T21P@3cx: Jerky Audio

We have some T21P E2-Phones connected to our (external, so behind firewall) 3cx (V.14), and some phones of other brands, which work fine.

There are some problems with all T21P E2, I can't get over...

Firmware-Version 52.80.0.95
Hardware-Version 52.0.0.0.0.0.0

The phones were autoprovisioned by the 3cx. The audio quality of external calls is bad - it's jerky. There is completely no audio in internal calls between two T21P and inbound audio is missing in internal calls between T21p and aphone of another brand.

Any ideas?

Thanks in advance!