You don't get 3G over GSM bands. Not sure how you got that from what I wrote.
With EDGE/GPRS data does not stop during a call.
You are thinking of CDMA. I certainly surfed while on the speakerphone on my Treo 650 and Centro many times.
Here's the deal: GPRS is a subset of GSM that sends data packets simultaneous to voice.
UMTS is also called WCDMA or wideband CDMA and also sends voice and data simultaneously.
When you are connected to a tower you are connected on a single frequency band, not two, and all your packets go over this band. You can be on one of four GSM bands or one of five UMTS bands (850/900/1700/1900/2100) depending upon where in the world you are located.
So if you have a tri-band UMTS (900/1900/2100) Pre3 in the US you will likely often drop to GSM for voice (when no 1900 tower is close enough) and you will get good old GSM hiss and pop. When you have a 1900 MHz tower you will have a much cleaner, WCDMA call.
Many people, myself included, force "2G" modes (tells phone not to use UMTS) to save power when we know it will be some hours before we can recharge as GSM is less power-hungry. You can also force 3G if you'd rather only have clean calls and faster data.