In short
A VoIP adapter, or analogue telephone adapter (ATA), is a small box that connects analogue phones and devices to a VoIP phone system. It plugs into your network, registers each analogue port as an extension and converts between analogue signals and SIP. It suits analogue handsets, door entry and paging; fax, lifts and alarms need extra care.
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What a VoIP adapter does
An analogue telephone adapter has two sides. On one side, a network port connects it to your router or office switch. On the other, one or more analogue phone ports, called FXS ports, provide dial tone and ringing to whatever is plugged in, just as a phone line from the exchange used to.
Inside, the adapter registers each analogue port with your VoIP phone system as an extension, and converts between the analogue signal and SIP and RTP, the protocols VoIP uses to set up calls and carry audio. To the phone system, the analogue device is just another extension; to the device, nothing has changed.
Some adapters also have an FXO port, which connects the other way round: to an existing analogue line or an old phone system's extension port. That is used mainly when linking older equipment during a migration.
What you can connect
| Device | Through an ATA? | Worth knowing |
|---|---|---|
| Analogue handsets and cordless phones | Yes | A simple way to keep familiar phones in shared areas or for staff who prefer them |
| Door entry and gate panels | Often | Test that the door release still works from the phone, as some panels rely on keypad tones |
| Overhead paging | Often | Cisco lists overhead paging among the devices its ATAs connect to Webex Calling |
| Franking machines | Sometimes | Older machines dial out over a phone line; ask your franking supplier, as newer machines may connect over the internet instead |
| Fax machines | With care | Fax is sensitive to how VoIP carries sound, and it depends on the platform. See below |
| Lift emergency phones and alarms | With great care | Life-safety lines need the maintainer's advice and testing. See below |
Some equipment can skip the adapter altogether. IP door phones, paging speakers and intercoms that register directly to the phone system appear on the supported lists for 3CX and Webex Calling, for example, and may be the tidier choice when the old panel is due for replacement.
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VoIP adapter or VoIP router?
People searching for a "VoIP adapter for router" are usually asking one of two things.
A separate adapter that plugs into the router. This is the standard ATA: a small box connected to a spare network port on your router or switch, with phone ports for the analogue devices. It works with any router, as long as the router is set up for VoIP.
A router with the adapter built in. Some routers have phone ports on the back, which is effectively an ATA inside the router. Digital landline services often use this approach, so a handset plugs straight into the router. Our explainer on the VoIP landline covers that, and our VoIP router guide explains the router settings that matter, such as turning SIP ALG off.
For a business VoIP phone system, a separate, supported ATA is usually easier to manage, because it can be provisioned and updated from the phone system like a desk phone.
Adapters on each VoIP platform
| Platform | Adapter support | Worth knowing |
|---|---|---|
| 3CX | 3CX's supported devices list includes PSTN gateways and ATAs, with FXS models for analogue devices and an FXO model | Choose an adapter from the list so 3CX can provision it |
| Microsoft Teams Phone | Microsoft's SIP Gateway compatibility list includes analogue telephone adapters from several makers | Each user or device needs a phone number with PSTN calling enabled. An analogue gateway connected through Direct Routing is another route |
| Webex Calling | Cisco multiplatform ATAs and voice gateways, plus listed third-party ATAs | Cisco says its ATAs connect non-IP devices such as analogue phones, fax machines and overhead paging, which then act as Webex Calling devices |
For Forever UC and other platforms, we can confirm the right adapter for your devices before you order. For Teams, our sister site explains Teams Direct Routing, the route used to connect a gateway for devices SIP Gateway does not cover.
Is a VoIP adapter the right fit for you?
We will check it against your systems and requirements and recommend the right option.
VoIP adapter for a fax machine
Fax was designed for analogue lines, and it is sensitive to the compression, delay and packet loss that ordinary voice calls tolerate. Whether a fax machine will work through an adapter depends on the platform and the route to the phone network:
- Microsoft Teams Phone: Microsoft says faxing services are not available to Teams customers through Phone System licences or any Microsoft Calling Plan, so plan an alternative for fax numbers on Teams
- Webex Calling: Cisco lists fax machines among the devices its ATAs can connect
- Other platforms: we can check fax support for your platform and trunk, and test it before you rely on it
Many organisations take the switch to VoIP as the moment to review fax: if few documents still arrive that way, an email-based fax service or retiring the number may be simpler than keeping a machine.
Lifts, alarms and other special lines
Lift emergency phones, intruder and fire alarm diallers, telecare and similar equipment are life-safety or security lines, and they need more than an adapter and a quick test. Before you move them:
- List every line and what is connected to it, including lines that rarely make a call.
- Ask the lift, alarm or telecare company what connection their equipment supports and what they recommend.
- Check any requirements from your insurer or monitoring centre.
- Plan for power cuts: an ATA needs power, so put it, the router and the switch on a UPS if the device must keep working.
- Test the device end to end after the change, and keep the old line until it passes.
Our sister site has a detailed checklist for alarm and special lines, and our switching from landline to VoIP guide shows where special lines fit in the wider move.
Setting up a VoIP adapter
- Count the analogue devices and where they are, so you know how many ports you need and where adapters will sit.
- Choose an adapter from your platform's supported list.
- Connect it to the network and to power, with a UPS for anything that must work in a power cut.
- Provision each port as an extension on the phone system, with the right number and call routing.
- Test every function: making and receiving calls, door release tones, paging, and any dial-out the device performs.
- Label the adapter and its ports, so the next person knows what each one serves.
Old phone systems with their own digital handsets are a different case: those handsets do not work through an ATA. If you want to keep them, see ISDN to VoIP for keeping the phone system on SIP trunks. Adapters, like desk, cordless and conference handsets, can be bought outright or, in some cases, included in your monthly rental, with the options confirmed at quote. Contact us with a list of the analogue devices you need to keep.
How Forever Systems helps you choose a VoIP phone system
We supply and support several VoIP platforms, so we can recommend the one that fits how your team takes calls rather than fitting your business around one product.
- Platform advice across hosted 3CX, Microsoft Teams Phone, Webex Calling, Forever UC and Forever Teams, with SIP trunks for phone systems you already own
- A requirements check of your broadband, network and routers before anything goes live
- Numbers and porting: we can help you keep your existing business numbers and plan the move away from analogue and ISDN lines
- Handsets and headsets that suit each platform, bought outright or, in some cases, included in your monthly rental, with the options confirmed at quote
- Set-up and changes: call flows, users and devices configured for you, and help with moves, adds and changes afterwards
- For BT Cloud Voice, see our sister site cloudvoice.cloud, and for hosted and cloud platforms in depth, cloudphone.cloud
We work with businesses, schools, charities and public sector organisations.
Frequently asked questions
What is a VoIP adapter?
A VoIP adapter, or analogue telephone adapter (ATA), connects analogue phones and devices to a VoIP phone system. It plugs into your network, registers each analogue port as an extension and converts between analogue signals and VoIP.
Can I use my landline phone with VoIP?
Yes, an analogue landline phone can work on a VoIP phone system through an analogue telephone adapter, which gives it dial tone and its own extension. Some routers also have phone ports with an adapter built in.
Will my fax machine work with a VoIP adapter?
It depends on the platform. Cisco lists fax machines among the devices its ATAs connect to Webex Calling, while Microsoft says faxing is not available through Teams Phone licences or Calling Plans. Fax is sensitive to VoIP, so test it, or consider an email-based fax service.
Can a lift or alarm line use a VoIP adapter?
Only with the advice of the lift or alarm company and careful testing. These are life-safety or security lines, so ask the maintainer what they support, plan for power cuts and keep the old line until the new connection has passed testing.
Do I need a VoIP adapter for my router?
Only if your router has no phone ports and you want to keep analogue devices. A separate adapter plugs into a spare network port on the router or switch. Routers with phone ports have an adapter built in.
Which adapters work with Microsoft Teams?
Microsoft's SIP Gateway compatibility list includes analogue telephone adapters from several makers, and each device needs a phone number with PSTN calling enabled. For devices SIP Gateway does not cover, an analogue gateway can connect through Direct Routing.
Still have a question about a VoIP adapter?
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