WiFi While on the Phone?

[Count_Chockula]

I have been experiencing some issues that I was hoping someone could straighten out for me. (yes I have done a search...)
Anyway,
When connected to a call on my phone (Evo--rooted) AND connected to a Wi-Fi signal I am unable to surf the web, get mail, or any do anything that requires a data connection. Why is this? In theory I should be able to do all of the above and more... right?
So, I have called a family member (who works for T-mobile) and she suggested that it was because Sprint has blocked that feature via the Rom. I didn't think that this was possible, given that Sprint has no "right's" to tell me when I can/ cannot use my Wifi connection. Anyway, the question that I would like to you smart people to answer is...
Why can I not use my WiFi connection (for any data purposes) at the same time as making a phone call. (I am not referring to the 3/4g connection...)