Is UUS (user to user signaling) supported?
Yes, isdn4linux could support both sending and receiving, but the implementation is currently rather incomplete due to the unclear legal situation for using this facility. Receiving UUS is only possible through the debug interfaces. Sending is supported in connection with the diversion services (when rejecting a call or announcing a busy condition), but not on an ordinary call. It is recommended to use subaddressing (see question feature_subaddressing) instead. Please note that sending UUS it is not a free service (receiving is free), at least with some German phone providers you have to pay extra for it (also have a close look on the usage conditions). Additionally, please note that if you are connected through a PBX, it may filter out all the UUS stuff.