Hi Emilymb00,
I sure hope it will remain to work out well for you.
I do recognize your steps. But then without the success.
When I received my 1st iPhone7 I installed it from my previous iPhone6S back up. When I had it in repair the very first time via Vodafone, I received a loan iPhone6 for the meantime.
I installed my latest iPhone7 backup to the loan iPhone6 and all worked perfectly. I tried and tested all problematic issues that occured with the iPhone7 and the loan iPhone6 worked perfectly. So I was assured the problem was with that iPhone7, not with corrupted or damaged software or so.
Then when I got the 1st iPhone7 returned (without repair as the 3rd party Vodafone repair group checked only the hardware via software testing and said all worked fine) I installed the iPhone6 backup to the iPhone7 again and instantly I had the same problem again with this iPhone7.
That's why I don't believe the 'corrupted file backup theory' so much. But it has been suggested to me as well in a later stage of problem solving by the Genius bar. Actually during my 2nd visit.
During my 1st visit at the Genius bar It was changed for an all new iPhone7 in Amsterdam, and I installed the 2nd iPhone7 from the 1st iPhone7 backup (that was turned in). The problem was still there with the new 2nd iPhone7 after it was all up and running (when restored from my 1st iPhone7 icloud backup).
During my 2nd visit to the Genius bar one week later, a Genius bar guy agreed that I did do almost everything imaginable. Even got a new sim card from Vodafone before. And the exchange for a new iPhone7 should have fixed it if the hardware was faulty of the 1st one.
He only suggested - as a last option - to do a DFU reset (cleaning the iPhone from any previous software installations and force it to install it as a new 'virgin iPhone') and not make use of any icloud backups. He personally did the DFU procedure while connected to a macbook at the Genius bar. Then I needed to 'rebuild' it from scratch. Needless to say that this was quite a journey as I had to install a lot of software 'manually' and enter passwords and usernames and all for different apps as I was not using the advantage of the bundled icloud backup or password manager.
Unfortunally I was not rewarded. After going through that all, the all new 2nd iPhone7 still had the same problem while being used for calls et cetera.
As I have walked a similar road but unfortunately with no success, I do hope that it's now really permanently solved for you.
Please keep us updated over the next weeks or so if you have no issues anymore or when you gained additional experience and insight. We could all well benefit from that. Thanks.