CAPI was an Application Programming Interface that allowed for the remote use of an ISDN device (e.g. fax server). The company TE-Systems designed a drop-in replacement for programs to still use the CAPI but instead of using an ISDN device the XCAPI uses SIP.
In the NFON PBX XCAPI describes a SIP trunk with registration (the SIP account (K…)) in the SIP From: header and the extension number (e.g. 306) in the To: header. Contrary to "normal" SIP trunks the device on the other end of the XCAPI trunk needs some PBX intelligence to be able to route the incoming calls accordingly.
CAPI was an API to use ISDN features over a computer network, i.e. a single ISDN connection could be used by various devices over a computer network (typically Ethernet and IP, but previously also other protocols such as IPX).
XCAPI is a product from te-systems to offer a virtual CAPI connection, i.e. even without being able to use the ISDN features as a drop-in replacement without an ISDN connection. This is often used for fax servers that have previously used an ISDN router to send faxes and have placed them on the CAPI interface. The product can now speak in the direction of the "CAPI" network and in the direction of PSTN ISDN, H.323 and SIP.
If we (NFON) speak of XCAPI connection, we are "the PSTN" from the view of the end device. Conversely, this means that the XCAPI device on the other end registers with us as an end device (namely at our registrars). The device is therefore not a simple end device, but from our point of view a small, independent PBX. It must be able to route numbers with it. The extension to be called is the P-asserted-identity header.
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