default or selected kernelversion does not have config value CONFIG_HISAX_ST5481.
Result is shown for kernelversion 6.8.2

ST5481 USB ISDN modem

modulename: hisax_st5481.ko

configname: CONFIG_HISAX_ST5481

Linux Kernel Configuration
└─>Device Drivers
└─>ISDN support
└─>Old ISDN4Linux (deprecated)
└─>Passive cards
└─>ST5481 USB ISDN modem
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
This enables the driver for ST5481 based USB ISDN adapters,
e.g. the BeWan Gazel 128 USB

source code:
drivers/isdn/hisax/