default or selected kernelversion does not have config value CONFIG_HISAX_BKM_A4T.
Result is shown for kernelversion 6.9.9

Telekom A4T card

configname: CONFIG_HISAX_BKM_A4T

Linux Kernel Configuration
└─>Device Drivers
└─>ISDN support
└─>Old ISDN4Linux (deprecated)
└─>Passive cards
└─>Telekom A4T card
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
This enables HiSax support for the Telekom A4T card.

See <file:Documentation/isdn/README.HiSax> on how to configure it
using a different D-channel protocol, or non-standard IRQ/port
settings.