Set up Groundwire / Acrobits Softphone with a NumberGroup SIP trunk

Acrobits (Czech Republic) makes the SIP softphone stack we ship to NumberGroup Members. There are three flavours of the same underlying app:

• NumberGroup branded softphone — our white-labelled Acrobits build. Push-notifications and CallKit pre-configured for our SIP gateway. Approx 75p per active install with Membership.
• Groundwire— Acrobits' commercial iOS/Android softphone (paid app, one-off £10ish). Same engine as our branded build but with the Acrobits brand.
• Acrobits Softphone — older free version with limited features. Works but no push, no CallKit.

This guide covers manually configuring Groundwire or Acrobits Softphone against a NumberGroup trunk. If you've got our branded build the SIP settings are already in place — you just enter the Authorisation ID and password and you're done.

When you ask us to set up the trunk, mention you want the User-Agent Type to "RFC3261 Standard Trunk" and Send Numbers As to "United Kingdom".

If you have the NumberGroup branded softphone

• Install the app from the App Store / Play Store using the link we email you
• Open the app → first-time setup prompts for Authorisation ID and password
• Enter the trunk's T-AAAAAA-NNN and password from the email we send you
• Done — push notifications, CallKit and codec settings are pre-configured

Stop reading. The rest of this guide is for users on Groundwire or the off-the-shelf Acrobits Softphone who need to configure manually.

Step 1: install Groundwire

• iOS: App Store, search Groundwire — paid one-off (~£10)
• Android: Google Play, search Groundwire SIP Softphone
• Desktop: Acrobits don't do a desktop build — for Mac/Windows use Zoiper or Linphone

Step 2: add a SIP account

Open the app, go to Settings → SIP Accounts → + (add).

• Choose Add Generic SIP Account (not the Cloud Softphone provider list)
• Title: NumberGroup
• Username: T-AAAAAA-NNN (e.g. T-102377-001)
• Password: your trunk password
• Domain: sip.numbergroup.com
• Auth Username: same as Username
• Proxy: leave blank

Step 3: advanced settings

Tap the account → Advanced Settings:

• Transport: UDP
• NAT Traversal: Auto
• STUN Server: leave blank
• Use Push Notifications: Yes (Groundwire's paid push works without further config)
• Registration Period: 600s
• DTMF Format: RFC2833

Step 4: codecs

Settings → Audio → Codecs:

• Enable, in order: PCMU, PCMA, G.722
• Disable Opus, iLBC, GSM, Speex — most PSTN gateways won't negotiate them

Step 5 (iOS): CallKit integration

Groundwire supports Apple CallKit out of the box. With it on, incoming SIP calls appear on the iPhone lock screen exactly like a normal call. Configure under Settings → Calls → Call Integration → On.

Allow notification permission and Background App Refresh in iOS Settings.

Step 5 (Android): battery optimisation

Android aggressively kills background apps. To stop the SIP registration dropping:

• iOS Settings analog: Phone Settings → Battery → Battery Optimisation → All Apps → Groundwire → Don't Optimise
• Allow notification permission
• On Huawei/Honor: disable PowerGenie and add Groundwire to the protected apps list — these vendors have aggressive vendor-specific battery managers

Step 6: call recording (optional)

Groundwire ships with built-in call recording as a pay-once in-app bolt-on (~£10). Recordings save locally to the device. For UK businesses recording calls, see our call recording guide — there are GDPR/PECR rules that apply.

If you need server-side recording for compliance, route calls through a hosted PBX with recording instead — included free with Membership.

Step 7: verify

• Account status shows green / Registered
• Portal's trunk page shows registered
• Ring your mobile to test outbound + Caller ID
• Lock the phone, ring the DDI from another phone, confirm push wakes Groundwire and CallKit rings

When it doesn't work

Registration fails

• Wrong Username or Password — re-check the email we sent with your trunk credentials
• Domain mistyped — must be sip.numbergroup.com
• Mobile network blocking SIP — try Wi-Fi or another mobile network. Some MVNOs throttle SIP.

Calls don't ring when phone is locked

• iOS: confirm CallKit is enabled and Background App Refresh is on
• Android: confirm battery optimisation is off for Groundwire
• Push notifications require Groundwire (paid) — Acrobits Softphone free version doesn't support reliable push

One-way audio

NAT issue. Set STUN Server to stun.acrobits.cz and NAT Traversal to Force RPort.

Set fraud caps

Mobile softphone — Fraud Detection Simultaneous Calls = 2 per registration. See our telecom fraud guide.

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