default or selected kernelversion does not have config value CONFIG_HISAX_1TR6.
Result is shown for kernelversion 6.9.9

HiSax Support for german 1TR6

configname: CONFIG_HISAX_1TR6

Linux Kernel Configuration
└─>Device Drivers
└─>ISDN support
└─>Old ISDN4Linux (deprecated)
└─>Passive cards
└─>HiSax Support for german 1TR6
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
Say Y or N according to the D-channel protocol which your local
telephone service company provides.

1TR6 is an old call control protocol which was used in Germany
before E-DSS1 was established. Nowadays, all new lines in Germany
use E-DSS1.