Use a UK business number on iPhone that rings like a native call (CallKit explained)

A common ask from new Members on iPhone: "Can I just add my SIP credentials in iOS Settings, like an email account, and have the native Phone app ring for incoming calls?"

The honest answer is no— iOS doesn't have a native SIP stack you can configure through Settings or an Apple Configuration Profile. Old web tutorials claiming this work refer to long-removed features in iOS 3 / 4 (the original VoIP framework predating CallKit). Don't waste time trying to install a SIP profile through Apple Configurator — there's no such payload in modern iOS.

The correct answer is CallKit, and it's actually better than what people are asking for.

What CallKit does

CallKit is the Apple framework SIP apps use to integrate with the iPhone's call UI. When a SIP app implements CallKit properly:

• Incoming SIP calls ring on the lock screen with the full-screen interface, exactly like a cellular call
• The call appears in the iOS Recents tab of the native Phone app
• iOS Contacts can be matched against the incoming number, so "Sarah Smith" appears instead of +441234567890
• Do Not Disturb respects the call
• You can answer with AirPods, CarPlay, Apple Watch — same as any cellular call
• The user has no idea (or doesn't care) it's a SIP call

That's what people actually mean when they ask for "native SIP". CallKit + push notifications = an iPhone experience indistinguishable from a normal call. The SIP app stays backgrounded; iOS wakes it via push when a call comes in.

SIP apps with proper CallKit support

Five we've tested against NumberGroup trunks:

• NumberGroup branded softphone (Acrobits) — pre-configured CallKit + push, you only enter Authorisation ID and password. ~75p per active install with Membership. The path of least resistance.
• Groundwire (Acrobits) — paid one-off (~£10). Same engine as our branded build. See our Groundwire setup guide.
• Linphone — free open-source, GPLv3. CallKit support built in. See our Linphone setup guide.
• Zoiper 5 Premium— paid (~£8 one-off). Free version doesn't support CallKit or push, so won't ring when the app is backgrounded — skip the free build for business use. See our Zoiper setup guide.
• Bria Mobile (CounterPath)— paid subscription. Enterprise-grade. CallKit and push are standard. Worth considering if you're an enterprise already standardised on CounterPath.

Apps to avoid for serious business use: any SIP softphone that doesn't advertise CallKit and push (most free builds, Acrobits Softphone free, older builds of Zoiper). They'll work as a desktop-style app while open, but the phone won't ring when locked.

The setup path

Pick one of the apps above and follow its setup guide. You'll need:

• Your trunk's Authorisation ID (T-AAAAAA-NNN) and password — both from the portal
• The trunk's User-Agent Type set to RFC3261 Standard Trunk
• The trunk's Send Numbers As set to United Kingdom (so incoming Caller ID matches iOS Contacts in the format 01214001262)

iOS settings to check

Regardless of which app you pick:

• Settings → [Your App] → Notifications → Allow Notifications, Sounds, Banners, Lock Screen
• Settings → [Your App] → Background App Refresh → On
• Settings → [Your App] → Microphone → On
• Settings → Phone → Silence Unknown Callers → consider turning off, because SIP calls from new numbers will be treated as "unknown" unless CallKit matches them against your Contacts

Showing "business" vs "personal"

A practical concern when you have one iPhone serving both: how do you tell business calls from personal calls without answering?

• Different ringtones. Most SIP apps with CallKit let you set a custom ringtone for the SIP account specifically. Set a distinct one for business so you know before you look.
• Caller ID label. CallKit shows the app name ("via Groundwire") below the caller name on the incoming-call screen. Easy way to tell it's the business line.
• Direct vs main number. If you have a switchboard plus DDIs, point the DDI at your iPhone's SIP app and the main switchboard at an IVR / hunt group. People with your DDI get straight through; everyone else gets triaged.

Same number, multiple devices

The same NumberGroup SIP trunk can register from your iPhone, an iPad, a laptop softphone and a desk phone simultaneously. Incoming calls ring all of them in parallel — first to answer takes the call. Useful if you want the office phone to ring at the desk while you're out, with the same call also ringing your iPhone two seconds later.

Why doesn't Apple just support native SIP?

Apple deprecated the old VoIP framework in iOS 12 and now requires apps to integrate via CallKit + PushKit. The architectural reason is battery life — a persistent SIP registration in the background eats power. Push wakes the app only when a call comes in, which is much kinder to the battery.

The trade-off is that there's no "just add SIP in Settings" option, and there probably never will be. The CallKit + push model is now the standard.

Source: the device navigation in this guide follows Apple Platform Deployment Guide (configuration profiles). We keep the click-path with the manufacturer so it stays current — the NumberGroup trunk values above are the part specific to your account.