Possibly. AT&T may not have a portability agreement set up with Vonage. I am going through a similar problem, trying to port my number from Cavalier Telephone (a Richmond, VA company) to AT&T's CallVantage.
I was told by AT&T that they could not port my number directly and suggested I switch from Cavalier to Verizon and then to AT&T. (To make matters worse, I cannot port my number from Cavalier to Verizon until I have the DSL removed from my line, which, according to Cavalier, takes "15 to 20 business days".) I think it is unfair to "use" Verizon in this manner; certainly they lose money setting up accounts for all these folks who just want to keep their home or business phone numbers, only to have them closed after a month or so of service. We all have been paying a fee for quite a while to insure number portability, and when we want to take advantage of it, the system fails.