Geronimo- Listen to me as I have the Thunderbolt. Here is a factual answer, not a guess.
I can make a phone call and receive a phone call while surfing the web, download e-mail, and even stream video using sling media. The later gets a bit challenging as the sound is a mix between the phone call and the TV Program

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But you can only do this when in a LTE region. If you operate the Thunderbolt outside the LTE region, you will be on EVDO Rev A and now your internet connection must be done via a #77 phone call. ALL CDMA / EVDO service is phone call based. Just thyat when you connect with EVDO, you dial #77 to connect to the web server. This is the reason why you can't do phone call and surf the internet at the same time. You phone can't call two different numbers at the same time. With LTE, the internet connection is it's own radio signal, similar to clearwire or a wifi RF signal that has nothing to do with a phone modem. So, you can operate both transceivers simultaneously.