Apart from much higher speeds, one of the advantages of UMTS networks compared to GSM is that voice and packet switched data can be transferred simultaneously. That comes in quite handy, for example, if you are stuck in a long conference call, as you can still send and receive e-mails and do other things when the conversation touches topics which are not in your area of interest. In GSM this is not possible in most networks.
Many years ago an enhancement was standardized in 3GPP, though, referred to as Dual Transfer Mode (DTM) to catch up with UMTS. Nokia and other manufacturers have supported this extension for years but I have never seen a network that would actually support it. Then, earlier this year I read in a support forum that an operator in the UK has activated DTM. Needless to say I was very much looking forward to my next trip to the UK to try it out.
It has taken half a year but I finally made it to London and gave it a try. Very nice it works as designed. While in a voice call, I get data rates of around 120-140 kbit/s which is only slightly less (one EDGE timeslot I presume) compared to data transfer rates while no voice call was ongoing. Well, done, I hope it's a growing trend!