Okay I am sure that is why. If you want to fix this problem, stop using internet (IM, refreshing widgets, browsing). Or you can tell the people who call you to leave a voicemail.
If not, then you have to switch to T-Mobile to get 3G for simultaneous data and voice. With 3G, you can use the internet and call at the same time. With 2.5G, you have to turn data off to call someone. Try browsing a page, then use the phone to call someone, the phone will pop up a message saying "Cellular data unavailable" or something like that.