Due to the fact that the uplink and downlink timeslot usage was not co-ordinated between the voice and data connections, the class A devices in practice had to be capable of transmit and reception simultaneously.
With DTM, a mobile phone capable can be engaged in both CS and PS call and simultaneous voice and packet data connection in GSM/EDGE networks with the lower Class B requirements thus having a simpler half duplex circuitry due to co-ordination of the timeslot allocations in uplink and downlink for both voice and data.
Older devices in Class B without DTM capability could still receive CS calls while having an active PS session.
In case a voice is started on a mobile phone that is in Packet Transfer Mode(i.e. in a data call), the procedure takes place in three stages.
This enables continuous data transfer also when calls are set up and released, as well as reduced load on the common control channels of the GSM network.