Connect Linphone softphone to a NumberGroup SIP trunk

Linphone is the free, open-source softphone from Belledonne Communications. Genuinely free (GPLv3, no "upgrade for push notifications" trap), available for Windows, macOS, Linux, iOS and Android, and respectable quality. Solid choice if you don't want Zoiper's paid model.

Set the trunk's User-Agent Type to "RFC3261 Standard Trunk" and Send Numbers As to "United Kingdom" on the trunk — incoming Caller ID then reads as a familiar UK number on screen.

Step 1: install Linphone

• Desktop: download from linphone.org (Windows / macOS / Linux)
• iOS: App Store, search "Linphone"
• Android: Google Play or F-Droid (latter is the truly open-source build)

Step 2: add the account (Use SIP account)

On first launch, choose Use a SIP account (don't pick "Use linphone.org account" — that's their own service, not ours).

• Username: T-AAAAAA-NNN (e.g. T-102377-001)
• Password: your trunk password from the email we send you
• Domain: sip.numbergroup.com
• Display name: your business name (optional)
• Transport: UDP

Hit Login. Within a few seconds the top-bar icon should switch to green / a check mark.

Step 3: codec preferences

Open Preferences (desktop) / Settings (mobile) → Audio:

• Enable, in order: PCMU (G.711µ), PCMA (G.711a), G722, Opus
• Disable iLBC, GSM, Speex unless you have a specific reason
• Echo cancellation: On
• Adaptive rate control: On

Opus gives the best quality if you're calling between Linphone endpoints; G.711 is the universal fallback that works with every PSTN gateway.

Step 4: network settings

Under Settings → Network:

• NAT and Firewall → Direct connection (no firewall) — if Linphone has internet access this just works. STUN/ICE are only needed in odd corporate-NAT scenarios.
• Random ports: disabled (use fixed 5060)
• Allow IPv6: disabled (keeps things predictable)

Step 5 (mobile): keep notifications working

Linphone Mobile is genuinely free including push notifications — that's a real advantage over Zoiper's paid model. But you do need to:

iOS

• Allow notification permission on install
• Enable Background App Refresh for Linphone in iOS Settings
• Linphone supports CallKit — incoming SIP calls appear like normal phone calls on the lock screen. Enable in Settings.

Android

• Disable battery optimisation for Linphone (Settings → Battery → Battery optimisation → All apps → Linphone → Don't optimise). Without this, Android kills the background SIP registration after a few hours.
• Allow microphone and notification permissions
• Linphone Android uses Firebase Cloud Messaging for push — works fine on most modern handsets including non-Google Chinese Android (Huawei, Honor) via fallback foreground service

Step 6: verify

• The status bar should show Registered with a green icon
• We confirm the trunk has registered our side
• Ring your mobile to test outbound + Caller ID
• Ring your DDI to test inbound

When it doesn't work

Registration fails

• Wrong username/password — copy from the email we sent with your trunk credentials
• Domain mistyped — must be sip.numbergroup.com
• Firewall blocking UDP 5060

Outbound 403

Default CLI on the trunk isn't a valid DDI. Ring us, we'll fix it.

One-way audio

Disable SIP ALG on the router. On corporate Wi-Fi, you may need to enable Linphone's STUN setting and point it at stun.linphone.org:3478.

Calls don't ring on Android when phone is locked

Battery optimisation is killing the SIP registration. See step 5.

Set fraud caps

Single-user softphone should never have more than 2 simultaneous calls. Tell us when you ask for the trunk and we'll lock that down in the Fraud Detection settings. See our telecom fraud guide.

Source: the device navigation in this guide follows Linphone documentation wiki. We keep the click-path with the manufacturer so it stays current — the NumberGroup trunk values above are the part specific to your account.