Are you sure you weren't on WiFi?
I just started a YouTube on my iPhone (on EDGE) and called my iPhone number from my work phone, and it rang 4x and then went to voicemail. At no time did my iPhone display the incoming call.
I pressed the "HOME" button on the iPhone (to return to the main menu) and then called my iPhone from my work phone, and it rang.
I have said EDGE/GSM can support an interrupt. He doesn't have to be on wifi for it to support that.
He's right as far as the UMTS/HSDPA goes.