How exactly do they block it? According to the verification instructions[0], Google Voice calls you and asks you to enter a verification code. How can this be blocked? It would be like any other DTMF service. Also, isn't it illegal to block this?
0: http://support.google.com/voice/bin/answer.py?hl=en&answ...
A lot of Google Voice numbers appear in a number pool marked for prepaid cellphone plans and a number of services will blacklist such numbers for use for identity verification. Sad but true.
You still have your cell phone provider's number and you should just use that to verify.
While there is a certain logic to this system historically, I don't think it makes a lot of sense anymore as prepaid cellular is quite popular and getting more and more so.
Why would they care?