Can't make outgoing calls using iPhone on iPad
I have an iPad running iOS 9.3.6 and an iPhone SE running 15.2.1. I'm in the UK.
I am trying to use Continuity to make and receive calls on my iPad. The hardware and software meet the requirements. I set up everything as instructed:
-- phone settings allow calls on nearby devices, then specifically allow on iPad
-- iPad Facetime settings make calls from iPhone
-- Facetime active signed in on same Apple ID on both devices, checked multiple times
-- both devices signed into same Apple ID
-- both devices on same WiFi network.
I know that Continuity is working because I receive incoming phonecalls on my iPad as expected, and I can answer them and talk using Facetime.
But I can't make outgoing calls on the iPad. When I select a contact in Facetime it gives me the option 'Start a phone call using your iPhone' as expected. When I select the number it says 'calling using iPhone'. Then the call immediately ends with the message 'Call Using iPhone Failed'.
I have already looked at tens of answers on a load of different sites and nothing helps. I've tried restarting both devices; toggling airplane mode off and on on both devices; signing out of my Apple ID and back in again on both devices. My iPad is 1st gen and can't be updated beyond iOS 9 but Continuity is supposed to work on that operating system and it does work for incoming calls.
Does anyone have any ideas beyond the ones I've already tried?
iPad mini, iOS 9