When credit balance exceeds $300 on my AT&T wireless account (currently has only 1 line), I get error messages that they cannot take in more payments. This is both while signed in or signed out.

On other hand I have paid for someone else’s 6 year old account (with atleast 4 lines) where there credit balance is close to $1700 and can still take in payments. And its not a business account. Any ideas on known factors that might be in play? like age of account, number of lines, or some other factor that can clue in on how to pay more in advance while the Chase offers for AT&T are active?