This is exactly why I don't want to use online banking!
You are making life difficult for yourself.
If you had already had online banking you would be conversant with how it all works. You wouldn't necessarily need a mobile app but it's simpler to use one.
Paying a new payee is easy. First of all you must ensure you have the correct payee details. Once you are sure, you simply use the app to enter the payee’s name, say whether it’s a personal of business account, input their account number, sort code and amount to be paid.
The bank will check that the account you are about to pay is bona fide.
You will be then be taken though a couple of security checks including two-step authentification via a text, email or card reader (depending on the bank) and it’s done. No phone calls necessary. Takes about five minutes tops.