I have seen a few GSM gateways around which enable PBX's to make calls over GSM networks using sim cards.

The DrayTek router has 3G usb dongle support for internet access.

I think it would be an awesome addition to the 2820 feature set if the 3g usb dongle could be utilised as a trunk to route calls over the GSM network via the attached sim card as well as being able to pass the calls to an attached phone for VoIP models or to the IPPBX system.

It can't be done; those dongles do not provide voice call facilities as I understand it, just data (and SMS).

if the dongle is capable of handling data/sms then sureley it can pass the call information to the attached router which after all are packets of data?

After all my signal shows a 3g connection on my mobile phone which i believe the voice services can be provided over. I do not believe my mobile phone maintains a seperate voice only connection to the mobile network otherwise why would voice services deteriorate in lower signal 3g areas?