If you have a FritzBox or a self fritzed box, then you may want use its genius functions like call monitoring and fax sending and receiving.
To have all that comfort really greatful on your Mac’s screen, I absolutely recommend the installation of the Fritz.Mac Suite. Unfortunately it costs almost 30 Euro, but keep in mind that you use it every day! You can download a full working trial.
Fritz.Mac Suite
The software communicates via different TCP ports with the FritzBox itself to have all that live data.
In my case the Box did not have the required ports for CAPI and Fax services open. This can be done by simply dialling some codes with the phone.
#96*3* to enable CAPI over TCP
#96*5* to enable Fax support
My problem was, that I could not dial those numbers, because I am using a VoiP enabled handset. The Fritz.Mac Suite contains a Call Manager which can dial for you. Putting the codes in there, solved the issue.