Telecoms glossary • M
Mobile VoIP
Your business extension on a phone in your pocket. The feature list is the easy part; what breaks it is the handover between Wi-Fi and mobile data.
What is mobile VoIP?in VoIP and business telephony
1. Mobile VoIP is placing and receiving calls over an internet connection from a smartphone rather than over the cellular voice network. In business use it means a softphone app that registers to your phone system as a normal extension, so calls to your business number ring on the handset and outbound calls present that number rather than a personal one.
2. The distinction that matters is which network carries the call. A mobile VoIP call travels as data over Wi-Fi or mobile data and is billed by your voice provider. A normal cellular call travels on the operator’s voice network and is billed by the operator.
How a mobile extension works
The app is a softphone. It registers to your phone system over SIP exactly as a desk phone does, so the platform treats it as an extension rather than as a diverted mobile number.
That is the whole point. A call forwarded to a mobile leaves the system, which means no transfer, no hold, no recording and no reporting once it has gone. A registered mobile extension stays inside the system, so transfers, hunt groups, call recording and reporting all still apply, and the presented caller ID is a company number rather than a personal one.
Registration also means the phone shows up in the platform’s presence data, so a receptionist watching a BLF key sees whether the person is on a call, wherever they are.
What it actually costs in data
Mobile data bundles count traffic in both directions, and a call sends audio continuously in both. Cisco’s per-call figures give the honest arithmetic, and the number depends entirely on which codec the app negotiates.
| Codec | Per call, each direction | Bundle used per minute | Trade-off |
|---|---|---|---|
| G.711 | 87.2 kbps | about 1.3 MB | Best quality of the narrowband options, heaviest on data. |
| G.729 | 31.2 kbps | about 0.47 MB | Roughly a third of the data, with an audible drop in quality. |
An hour of talking on G.711 is therefore around 78 MB of bundle, and on G.729 around 28 MB. Neither is a reason to avoid mobile VoIP, but both are worth knowing before someone spends a week on the road on a capped bundle. If your app lets you set a different codec preference on mobile data than on Wi-Fi, use it.
Where mobile VoIP breaks, and what to do
- Walking out of Wi-Fi range
- Moving from Wi-Fi to mobile data changes the handset’s IP address, which means the SIP session has to be re-established. Some clients survive it, many drop the call. Test it deliberately before you roll the app out, and if the platform supports pinning the app to mobile data only, that is often steadier than a handover.
- One-way audio on mobile data
- Mobile connections are almost always carrier-grade NATed, so the handset has no reachable public address and inbound media has nowhere to go. This is the provider’s problem to solve with STUN, TURN or a session border controller. Ask whether they do, because it is the difference between an app that works on any network and one that works only on the office Wi-Fi.
- The app going to sleep
- Aggressive battery optimisation stops the app registering, so calls go straight to voicemail and nobody knows why. Most vendors deal with this using push notifications rather than a permanently open registration. Ask which approach the app uses.
- Emergency calls
- A softphone does not present a physical location, and emergency routing from a data call is not something to rely on. Emergency numbers should be dialled from the handset’s normal cellular dialler, and staff should be told that explicitly.
The one thing mobile VoIP does exceptionally well is continuity. A power interruption at the office takes the desk phones down and leaves the mobile extensions ringing on cellular data, which is why it is the standard business continuity answer in South Africa rather than a marketing feature. Pair it with failover routing so calls follow the mobile extensions automatically instead of waiting for someone to reconfigure anything.
Ask before you sign
Ask whether the mobile app is included in the per-user licence or charged as a second device. Some platforms count every registered endpoint, so a staff member with a desk phone and an app costs two seats.
Then ask three technical questions: what happens to an active call when the handset moves from Wi-Fi to mobile data, whether the app uses push notifications or a permanent registration, and which codec it defaults to on mobile data. The answers separate a genuine mobile extension from a rebadged consumer dialler.
About this entry
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