default or selected kernelversion does not have config value CONFIG_GIGASET_UNDOCREQ.
Result is shown for kernelversion 5.3.7

Support for undocumented USB requests


Linux Kernel Configuration
└─>Device Drivers
└─>ISDN support
└─>Old ISDN4Linux (deprecated)
└─>Siemens Gigaset support (isdn)
└─>Support for undocumented USB requests
In linux kernel since version 2.6.20 (release Date: 2007-02-04)  
This enables support for USB requests we only know from
reverse engineering (currently M105 only). If you need
features like configuration mode of M105, say yes. If you
care about your device, say no.