Our devices have VoIP passthrough option, which means that the device can pass the traffic. The device is not able to act as a VoIP endpoint (eg.: mobile phone).
Most of the time calls on our devices are used for alert options (eg.: certain action execution once the router receives a call), on the other hand, our router also can execute calls, which can be done via the command line via the use of scripts to send a trigger, but voice/audio traffic would not be possible in this scenario. This exact scenario that you are referring to would potentially require hardware changes. Here’s a wiki article if you would like to read more about mobile utilities: RUT906 Mobile Utilities - Teltonika Networks Wiki