want to merge o2 mobile account with forum only account

First register the phone number without an email address.

I did but I am unable to tie it to my email because it is already registered

Why do you need a link to the email address? Everything works without it. Otherwise your community account must be deleted.

It’s just annoying and complicated having two accounts and passwords, and I’d prefer to have email more directly connected to my number and billing details

As written, there is no need for a link. The email address is in your contact details and is used for communication. You can log in to the customer portal using your phone number.