The incoming call has been successfully rejected. Caller will hear the busy tone. It work as expected, I assume. Your ISP has nothing to do with it - he should just forward any code your phone responded to caller.
You didn't described the issue you have so I wish you will be satisfied with answer "it's not possible to modify code used for user-defined user-busy response".
Mark thread as answered if it answered your questions. It will help others to found solutions.
It is possible to register multiple phones for the same credentials - but same credentials claim the same user on all phones. Such user can claim it's busy, but then, there is no reason to ring same user on other phones - the user claimed he is busy, so he is busy.
It seems you wish to have three independent phones instead. Then ask your provider for three pairs of credentials and three phone numbers. Well, you can have one more "group" number in advance assigned by your provider. Calling such group number will ring all numbers in a group - but they are independent, so busy status of one of them doesn't affect others.
But short answer is - you can't order SPA504G to announce busy mode transmitting a different other code from the one dedicated for busy mode. Sorry.