You can use the Siemens 3750 plugin.
Please see below documents on how to setup the PABX unit.
See PABX3 Plugin: Siemens Hipath 3750
NOTE:
The Siemens 3800 has a serial or IP option to receive call details from the PABX. The IP option requires an additional card to be installed to the PABX, therefore the serial option is used most often.
To receive call details from the IP version - see forum post: TNG and Siemens 3800 IP option