Hello Gerard,
The DTM feature (alcatel b10) allows to send or receive packets while being already in a CS communication.
During a CS call, one user can get PDCHs while still keeping its CS timeslot. Of course, the GPRS performances are a little bit reduced (if the MS is originally a 3+2 then, during a CS call, it can only get 2+1 PDCHs).
It doesn’t require hardware changes in the BSS nor NSS, it is a very “soft” change in the network.
Regards,