Connect a Snom M-series DECT phone to a NumberGroup SIP trunk
Map your NumberGroup SIP trunk credentials onto a Snom M300/M700/M900 DECT base — create a Server, add an Extension that uses it, then register a handset. Mapping table plus Snom's own admin guide.
This registers a Snom M-series DECT base (M300 / M700 / M900, with Mxx cordless handsets) to a NumberGroup SIP trunk. The M-series does it in two stages that are worth understanding up front: you create a reusable Server object (the registrar), then add an Extension that uses that Server and holds the credentials — and a handset registers against the Extension.
When you ask us to set the trunk up, request Send Numbers As set to "United Kingdom" so Caller ID arrives in the familiar 01214001262 format on the handset screen.
The credentials and settings your device needs to register against the NumberGroup SIP trunk are:
- Domain / registrar:
sip.numbergroup.com - Authorisation ID (also called SIP username): looks like
T-AAAAAA-NNNe.g.T-102377-001 - Password: a random string we generate per trunk
- User-Agent Type: we set this our side to match your device (e.g. 3CX Phonesystem, DDI in R-URI (Asterisk), RFC3261 Standard Trunk) so DDIs present in the format your equipment expects
- Send Numbers As: also set our side — pick the format that works best for your platform:
- International (+xx) — true E.164 (
+441214001262) for 3CX, Cisco, Yealink and most modern PBXs - International (xx) — bare digits (
441214001262) for Asterisk / FreeSWITCH dialplan regex - United Kingdom — UK format (
01214001262) for direct-register desk phones / softphones where humans read the Caller ID
- International (+xx) — true E.164 (
- Allowed IPs (optional): for an extra security layer we can lock the trunk to only register from your office IPs — tell us which when you ask
Once you have the credentials, the rest of this guide walks through the device-side config. If you'd rather skip the config altogether, ask us — we'll do it for you on most common PBXs and phones.
Field mapping
Snom owns the click-path and keeps it current — the table below maps each NumberGroup value to its field, across the two objects the M-series splits config into: the Server (created once under the Servers tab) and the Extension (added under the Extensions tab, which selects that Server).
Find the base's IP from any Snom handset (press the menu key and type 47 for the IP Search screen), browse to it (default login admin / admin), then: first the Servers tab → Add Server for the registrar, then the Extensions tab → Add Extension, selecting that Server and entering the credentials. Finally register a handset against the extension.
Full navigation is in the manufacturer's own guide: Snom M700 / M300 Admin and Installation Guide (PDF). We keep the click-path with them so it stays current — the table below is the part that's specific to your NumberGroup trunk.
| NumberGroup value | Snom M-series field |
|---|---|
| sip.numbergroup.com | the registrar/server address (Servers → Add Server) |
| select that Server | Server selection (Extensions → Add Extension) |
| T-AAAAAA-NNN | Authentication User Name (Extensions → the extension) |
| your trunk password | Authentication Password (Extensions → the extension) |
| your DDI (or T-AAAAAA-NNN) | Extension / number — the identity the handset presents |
| UDP, RFC2833, codecs | set on the Server / Extension per the admin guide |
Server first, then Extension. The Server is a reusable object you create once and then bind every handset's Extension to — it isn't a per-handset field. Skip it and the Extension has nowhere to register.
sip.numbergroup.com in the Registrar field — this is the reusable Server object every handset's Extension then binds to. The address pictured is the vendor's example — use sip.numbergroup.com.Verify
- The base's Extensions page shows the extension registered (green), and the handset shows a line indicator
- We can confirm the trunk has registered our side — ring us or check the trunk-live email
- Dial your mobile from the handset to confirm Caller ID; ring your DDI from a mobile to confirm inbound
If it won't work
- Extension won't register: confirm it has a Server selected, and the Server address is
sip.numbergroup.comexactly - Wrong Authentication User Name / Password — re-check against the trunk email
- Remember: each handset supports only one identity per SIP server — one Extension per handset
- Outbound UDP 5060 blocked on your network firewall
- Outbound rejected with 403: your Default CLI on the trunk isn't one of your DDIs — fix on the trunk
Set fraud caps before going live
A DECT base with several handsets should be capped to your real peak. Ask us to set Fraud Detection Simultaneous Calls and a sensible Daily Spend ceiling. See our telecom fraud guide.
Cordless that just works
Members get SIP trunks with no per-channel fee and BT Wholesale call rates passed straight through — run your DECT handsets across the site on a modern carrier without replacing the kit.