CDMA is slower to suspend the data session than GPRS, thus the perception that this condition is exclusive to CDMA.
This is correct.
On GSM devices, your calls go to voicemail whenever there's "data transfer" going on. Such as in the middle of loading page. However, if the call comes in while you're between webpages (no further data), it will come through. Basically, it's as if the data connection immediately connects/disconnects after every packet. (Zero overhead to switch between voice and data mode)
On CDMA devices, because of the way CDMA data work (lots of overhead to switch between voice and data mode), the data connection will idle for some time for efficiency's sake. So that means, for XX number of seconds after you finish loading data in a third party application, your calls will not come through until the device has idled long enough after the last packet (timeout).