Multiple account numbers associated with telephone numbers problem

Hello all, hope you are well.

I have an excel sheet which lists telephone numbers down one column and next to it an account number associated with each telephone number. These are all in order i.e. all the telephone numbers and account numbers are consecutive down the columns.

What I need to do is put the letter A in the field next to the first telephone number for each account and the letter B in the field next to the last telephone number for each account.

I imagine this is possible with some sort of IF statement and the EXACT function to compare the account numbers?

If anyone has any thoughts I would be extremely grateful to hear them, any input is greatly appreciated.

Kindest regards, Mike