iphone experts, help please

[ChrisS]

I'm on PAYG and had an iphone 5S for 2.5 years that worked fine with imessage and Facetime via my mobi number in 'send/receive'.

Now PAYG on iphone 7, IOS 13.1.3. I'd like to say I've tried everything but I'm hoping an iphone expert will have the definitive answer. Spent an hour, each, with Apple and Vodafone, neither can resolve and basically gave up by suggesting I stick with shown email addy.

There is credit on the phone. In 'send/receive' is my icloud email and, below, in black, my mobi number, and whilst there's a tick alongside the email addy, no matter what I do the handset won't accept 'tick' my mobile number. I click my mobi number, buffer wheel runs for a few seconds, then stops. Number remains black, never has been greyed out. Outgoing/incoming calls/texts/imessages received/sent, Facetime works (or so it seems). Tried:

Removed sim, reset back to factory settings, backed up from icloud,
signed in/out of icloud/appleid numerous times,
waited for 24 hours,
tried my mobi number in +44 and 07 format,
clicked on icloud in settings, turned off message to disable/upload messages,
reset network resources time and again,
turned airplane mode off/on numerous times,
checked and ensured time/date is correct and set to auto,
turned imessage and Facetime on/off, rebooted many times,
I'm sure I've tried other suggestions but can't remember!

If my mobi number had been greyed out below my icloud addy it appears to be an easy fix, but as it's black the blasted thing refuses to be acknowledged.

I'd be obliged for a resolution, or, if opinions are my icloud addy in 'send/receive' vs mobi number shouldn't hinder using Facetime or imessage, should I leave well alone? Thank you