default or selected kernelversion does not have config value CONFIG_ISDN_DRV_AVMB1_VERBOSE_REASON.
Result is shown for kernelversion 6.5.1

Verbose reason code reporting


Linux Kernel Configuration
└─>Device Drivers
└─>ISDN support
└─>CAPI 2.0 subsystem
└─>Verbose reason code reporting
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
If you say Y here, the CAPI drivers will give verbose reasons for
disconnecting. This will increase the size of the kernel by 7 KB. If
unsure, say Y.