Depending on the telco hardware, a phone can with
Default programming
at telco side)
Originate voice call
Originate Data call
Receive Voice call
Will not recive data call (page fail)
If its programed for data mode:
Originate voice call (cos 3)
Originate Data call (cos 4)
Receive Data call
Will not recive Voice call
Seems then that if you program one phone to be isp
it will not answer voice calls anymore.
This depends how packets are spread on the net.
For voice they are lpcded and so loose data integrity
For data, 2 channels are set, and no loss but no voice...