BlackBerry devices are GSM/GPRS enabled and use the same part of the network to work. The sims are provisioned so that when the device is turned on it makes a connection to the GSM network (voice) and then GPRS.
Sometimes GPRS can be lost due to a coverage issue, however you may still be able to make voice calls are you are still connected to GSM.
GSM has a stronger signal strength than GPRS. Most mobile operators have more network connections for GSM that GPRS.
I do not believe you can simply switch between the ability to turn off GPRS and just use voice or vice versa, however the mobile operator may be able to put some sort of bar, but you will have to constantly contact them to turn this on or off.
Hope this helps
:o)
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