VoIP Gateways Explained: FXS, FXO, GSM and E1, and How to Size Them
Every building has something that is not an IP phone: the fax machine the bank insists on, three hundred hotel room handsets, the lift phone, the gate intercom, the paging horn in the warehouse. A gateway is the small box that lets all of it keep working after you move to an IP PBX. Here is which type does what, and how many ports to buy.
- What a VoIP gateway is
- FXS vs FXO: the one thing people get backwards
- FXS gateways: keeping analogue phones and fax
- FXO gateways: keeping analogue lines
- GSM / 4G gateways: SIM trunks and outage backup
- E1 / PRI gateways
- How many ports do you need?
- Three mistakes that make gateway projects fail
- Typical UAE setups
What a VoIP gateway is
A gateway is a translator. On one side it speaks SIP to your IP PBX; on the other it speaks whatever the old equipment speaks: analogue telephone lines, analogue handsets, mobile SIM cards or an E1 circuit. It is a small rack or desktop box, and it is what turns a “we cannot move to IP because of the fax” conversation into a line item.
Four kinds cover almost everything you will meet in a UAE building:
| Gateway type | What it connects | Typical use |
|---|---|---|
| FXS | Analogue phones, fax machines, door and lift phones, paging amplifiers | Hotel rooms, factories, old desk phones, fax |
| FXO | Analogue lines coming from the operator | Keeping existing landlines during or after a migration |
| GSM / 4G | SIM cards | Cheaper mobile calls, SMS, backup when the fibre is cut |
| E1 / PRI | A digital E1 circuit (30 channels) | Large hotels and call centres with an existing PRI |
FXS vs FXO: the one thing people get backwards
Both are RJ11 sockets, they look identical, and half the confusion in gateway projects starts here.
- FXS provides a line. It supplies power, dial tone and ring voltage. Plug devices into FXS: analogue phones, fax machines, intercoms, paging amplifiers. FXS = Station.
- FXO consumes a line. It behaves like a telephone towards the operator. Plug incoming lines into FXO. FXO = Office.
FXS gateways: keeping analogue phones and fax alive
This is the most-used gateway in the region, because so many buildings are full of analogue handsets that work perfectly well.
- Hotels. Room phones stay; the gateway sits in the riser cupboard and the PBX handles wake-up calls, room status and billing through the PMS.
- Fax. One port per machine with T.38 enabled. Keep the fax number on the trunk it belongs to.
- Lifts, gates, car parks, cold rooms. Devices that are certified, cabled and not worth replacing.
- Paging and bells. A paging amplifier or a warehouse bell driven from an extension.
Sizes we stock: TA400 (4 FXS), TA800 (8 FXS), TA1600 (16 FXS) and TA2400 (24 FXS), plus onboard FXS modules for Yeastar P-Series such as the EX30 when only a couple of ports are needed.
FXO gateways: keeping analogue lines
An FXO gateway brings existing operator lines into the IP PBX. Three situations make it worth buying:
- Migration. You move to an IP PBX before the SIP trunk is provisioned, and the business cannot wait. The old lines keep working through FXO until the trunk is live.
- A line that must stay. Lift emergency lines, alarm panels, and landlines printed on ten years of company literature.
- Branch offices where a single analogue line is the cheapest possible connection and a SIP trunk is not justified.
Sizes: TA410 (4 FXO), TA810 (8 FXO), TA1610 (16 FXO).
GSM / 4G gateways: SIM trunks and outage backup
A GSM or 4G gateway puts SIM cards into your phone system. Two honest reasons to buy one in the UAE:
- Cost. If your staff call mobiles all day, routing those calls out of SIM cards on a mobile bundle can be cheaper than the same minutes on a fixed trunk. Model it with your actual call pattern before you buy: the saving is real for some companies and negligible for others.
- Resilience. When the fibre is cut, the PBX fails over to the SIM trunks and the company can still make and take calls. For clinics, hotels and any business that loses money in silence, this is the stronger argument.
Sizes: TG200L (2 SIM), TG400L (4 SIM), TG800L (8 SIM), TG1600L (16 SIM). Most offices need two to four channels for backup; call-heavy operations size on concurrent mobile calls.
E1 / PRI gateways
An E1 carries 30 voice channels on one circuit and was the standard trunk for large UAE hotels and call centres before SIP. An E1/PRI gateway converts it to SIP so a modern IP PBX can use it. Buy one when the circuit already exists and works; for a new installation, a SIP trunk is simpler, cheaper to scale and does not need the extra box. Ask us for current E1 gateway options if you have a PRI in place; they are ordered per project rather than stocked.
How many ports do you need?
Count devices for FXS and lines for FXO, then add spares. Ports are cheap at purchase and expensive to add later because the box has to be replaced.
| Situation | Count this | Buy |
|---|---|---|
| Office with a fax and a lift phone | 2 devices | 4-port FXS (room to grow) |
| Factory: gate, 2 workshop phones, paging | 4 devices | 8-port FXS |
| 60-room hotel keeping analogue room phones | 60 rooms + service phones | 4 x 16-port FXS, one per riser |
| Keeping 3 existing landlines during migration | 3 lines | 4-port FXO |
| Backup trunk for a 30-seat office | 2 concurrent calls | 2 SIM 4G gateway |
| Call-heavy sales team calling mobiles | peak concurrent mobile calls | 4 to 8 SIM gateway |
Three mistakes that make gateway projects fail
- The survey missed a device. The lift phone, the gate intercom and the fire panel line are never in the original list, and they are always discovered on cutover day. Walk the building and open the riser cupboards before ordering.
- Fax without T.38. Fax over IP fails intermittently if the PBX and gateway are not set up for it, and intermittent failures are the worst kind to diagnose. Configure it, then send ten test pages before handover.
- No labelling. A 24-port gateway with unlabelled ports is unmaintainable. We label each port to the device and hand over a port schedule; insist on that from any supplier.
Typical UAE setups
- Hotel, 120 rooms. IP PBX with PMS integration, FXS gateways in the risers for the existing room handsets, IP phones at reception and back office, DECT for housekeeping, 4G gateway as the outage backup.
- Factory or warehouse. IP PBX, DECT coverage across the floor, FXS gateway for the gate phone, the workshop handset and the paging amplifier, FXO to keep the old line that the alarm company uses.
- Clinic. IP PBX, IP phones at reception and nursing stations, FXS for the fax that the insurers still require, 4G backup so appointment calls survive an outage.
- Office moving premises. IP PBX and IP phones in the new office, FXO gateway keeping the old landlines alive during the overlap month, then removed.
Tell us what has to keep working
List the analogue devices and lines you cannot lose: fax, room phones, lift, gate, paging, old landlines. We come back within one working day with the gateway types, the port counts including spares, and the installed cost.
Questions we get asked
What is the difference between FXS and FXO?
FXS ports provide a line; FXO ports consume one. An FXS port is what a wall socket in your house does: it powers an analogue phone, gives it dial tone and makes it ring, so you plug phones and fax machines into FXS. An FXO port is what the back of a phone is: it connects to a line coming from the operator, so you plug incoming analogue lines into FXO. The memory trick: FXS = Station (the device), FXO = Office (the line).
What is a VoIP gateway used for?
Translating between analogue equipment and an IP phone system. It lets you keep analogue handsets, fax machines, door and lift phones and paging amplifiers on a modern IP PBX, bring legacy PSTN or PRI lines into that PBX, or add SIM-card trunks for mobile calls and outage backup.
Can I keep my fax machine?
Yes. Connect it to an FXS port and keep the T.38 fax setting enabled on the PBX, which makes fax survive the trip over IP. Two practical notes: give fax its own port rather than sharing, and keep the fax on the same trunk that its number belongs to. Many customers also add fax-to-email for incoming documents and keep the physical machine only for the counterparties who insist on it.
How do hotel room phones work after moving to IP?
Two ways. Either replace the room handsets with IP or Wi-Fi hotel phones, or keep the existing analogue handsets and feed them from FXS gateways, typically in 16, 24 or 32-port units in the riser cupboards. The second option is what most refurbishments choose: the rooms are untouched, and the PBX still handles wake-up calls, room status and billing through the property management system.
Are GSM gateways legal in the UAE?
Using SIM cards from a licensed UAE operator in a gateway for your own company calls is normal practice. What is not allowed is call termination: taking international traffic and pushing it onto local SIMs to bypass the operator. If someone proposes that as a way to “sell minutes”, walk away. Used properly, a 4G gateway is a cost and resilience tool, not a bypass.
What happens to my phones if the fibre is cut?
Internal calls keep working because the PBX and phones are on your own network. External calls need a second path: a trunk from the other operator, or a 4G gateway with SIM cards that the PBX fails over to automatically. For a clinic, a hotel or a call centre, that backup is worth the price of the box the first time it is used.
Do I still need an E1 or PRI?
Rarely for new sites, since SIP trunks are easier to scale. It matters where a building already has an E1 that works and a large hotel or contact centre depends on it. An E1/PRI gateway connects that circuit to a modern IP PBX, so you upgrade the phone system now and retire the E1 later.
Can one gateway do FXS and FXO together?
Yes. Several models mix both, and many IP PBX appliances have onboard slots for FXS or FXO modules, which is often cheaper than a separate box for small sites. On larger sites we use separate gateways so that the ports sit close to the equipment they serve, for example in the riser cupboard on each hotel floor.
Does a gateway degrade call quality?
Not noticeably when it is sized and configured correctly on a local network. Quality problems in gateway projects almost always come from three other places: an overloaded internet link, no QoS on the network, or trying to run fax without T.38. Fix those and analogue devices through a gateway sound the same as they did before.
Which brands do you supply?
Mainly Yeastar TA (FXS/FXO) and TG (GSM/4G) gateways, plus Grandstream equivalents, because they are proven with the PBX platforms we deploy and available from Dubai stock. We configure and label them before delivery, so each port matches the device it serves.
Written by the IRIX telephony team in Bur Dubai. Browse VoIP gateways, IP PBX systems, IP phones and PBX modules. All brand new with manufacturer warranty, configured and labelled before delivery.