It all depends on the operator. A UMTS (3G) network can accept users using a GSM SIM if the operator allows it. Authentication is performed by the network HLR

It all depends on the operator. A UMTS (3G) network can accept users using a GSM SIM if the operator allows it. Authentication is performed by the network HLR (Home location register) which is independent of the radio technology used. The procedure/algorithms are different for 3G-capable UEs with USIM, but the HLR can accept 2G users as well. Bottom line is that if your 'home carrier' (the one that produced the SIM) allows it, you can use your SIM in any 3G network that is part of the roaming agreement of that carrier.