Active calls needs to be terminated first. Terminate them from exchange's side. Or initiate power cycle on switch (if your ATA is PoE powered).

I'm not sure if call can be terminated by configuration, for example, by disable of particular line.

Unfortunately, the voice module isn't available at this moment. And in even a working condition it doesn't allow to switch off the line, and the system can't reboot with active calls. Well for nonsense. Reset on power helps, but it is inconvenient as devices are territorially carried.

It is necessary to execute cisco device reset with the current active calls. 

Then, analyze why it is necesarry to reset device with active calls on it and try to avoid such condition. No further ideas, sorry.