My account is now verified and there still isn’t any option in the app to change the phone number. I called them up and this time the advisor was very helpful, put me on hold to double check whether it’s possible to use the same number on two accounts (apparently yes) and said he’ll pass it on to the relevant team and it should be done after the weekend.
So overall this is a success! There definitely seem to be issues with their CS, I think they’re now growing much faster than they used to and are having trouble scaling the CS team to keep up. But overall I’m happy. This is the best business account I’ve ever used despite the minor annoyances. 
️
Now just got to build the FreeAgent integration with their API and it’ll be perfect!