On MP50s responding to some phone trees ends the call

[TRK45]

We have an issue where the call is terminated when using the keypad to respond to a call tree.

It doesn't happen on all call trees but when we find a number that has the problem all the MP50 phones have the problem.

So far, it only happens on the MP50 keypad.
The teams client works fine as do the MP54 and MP56 phones.

We've tried updating the firmware.
We've not been able to find anything like a configuration option that might help.

Any ideas are welcome.