Adding 2nd phone to account

I want to add my daughters phone to my account. It is a already activated phone and we use straight talk every month. I just need her on my account. The CS rep I talked to told me to wait till her end service day which was yesterdsy and then I could add it but everytime I try to it says wrong serial, wrong phone number. Or the worst this phone number isnt associated with this account.
Obviously I'm trying to add it. Very frustrating. Help 

