The number of voice calls is “fixed”, in the way that it is exactly the number of calls that are demanded by the subscribers.
There’s no way you can increase or decrease this number (unless you modify the radio coverage of the cell).
But you can change the radio rab voice / rab data. It is the number of RAB DATA that you can easily manipulate, by modifying the INACTIVITY timers that control the RAB release.
Increase the timer and less PS RAB will be needed (PS RAB will be reused, because data transmission is bursty and will reuse the same RAB if it wasn’t already released). The PS RAB duration will be much longer, and even though there are less PS RAB, the overall capacity used by PS RAB will be greater.
Decrease the timer and more PS RAB will be established, but on the other hand the capacity used by PS RAB will be reduced..