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.

Are you looking for more information? Here you can find our instruction manuals !

Further information

Virtual fax machine

With Cloudya, you can send and receive faxes.

  • Reach each employee directly via a personal fax number
  • Share virtual fax machines with a user group
  • Receive faxes in PDF format by email
  • T.38 fax…
Read more


SIP stands for Session Initiation Protocol. SIP is the universal standard for Voice-over-IP (VoIP) solutions because it allows telephone calls to be made over the Internet.
The voice functionality…

Read more

Private Branch Exchange (PBX)

Private Branch Exchange (PBX) is another term for a telephone system or an interphone network. It refers to a system connecting a specific number of communication devices such as phones or fax…

Read more

Session Initiation Protocol (SIP)

The acronym SIP stands for Session Initiation Protocol and refers to a TCP/IP-based network protocol which can be used to establish and control communication connections of several subscribers. SIP is…

Read more

Calling Line Identification Presentation (CLIP)

Using the feature Calling Line Identification Presentation (CLIP) it is possible for the call recipient to display the telephone number of the caller. Whilst CLIP was originally only available for use…

Read more