Did they resolve this issue for you? I have exactly the same problem. I called Sprint, they say everything is fin on their side, and after a remote phone reset it worked. One day later it stopped working again. I will cancel my plan over this and move my family elsewhere - it is incredibly annoying, and prevents me from reaching my parents.
I am here for you, and want to help! Are you getting any error messages when trying to intergrade, and have you tried doing it through Google.com/voice? Keep me posted, and let me know what you have already tried. I want to get this working for you!