In this article, we assume that you have access to the SIP traces of your phone system. One way to obtain those traces, is the Wireshark tool.

Please note that this topic requires advanced knowledge of VoIP and networking. If in doubt, please contact your network engineer.

Technical background

For establishing a VoIP call, one side sends a "SIP Invite" to invite another subscriber to join the conversation. In addition to the technical conditions, this also contains information about the calling subscriber. A separate SIP Invite is also created for call forwarding.

In order for this to be clearly identified by our infrastructure and for the call to be reliably transferred, it is crucial that the SIP user name of the call-initiating number is transmitted in the "SIP from address User Part" field - "From-User part" for short. If this does not happen, our infrastructure will reject the outgoing call attempt for security reasons.

Possible causes of failure

We regularly observe that systems are configured in such a way that, instead of the SIP user name, the User Provided Number (UPN) is transmitted in the From-User part, i.e. the call number that is to be displayed to the remote station

Please always use one of the following fields for transmitting the UPN:
Remote Party ID

Learn more about this matter in our article „Configuring CLIP no screening correctlyn“.

So ist es richtig

Here is an excerpt from a trace with a properly configured SIP invite. The key data of the call are as follows:

Number block: 030-80951 (000-999)
SIP user name: 00493080951
UPN: Im Feld From-Display (hier die 03080951055)

From: "03080951055" <>;tag=xxxyyyzzz
SIP Display info: "03080951055"
SIP from address:
SIP from address User Part: 00493080951
SIP from address Host Part:
SIP from tag: xxxyyyzzz

If the trace of your connections shows the data as in this example, you can exclude the configuration of the caller ID as the cause for failure.

If not, please check the settings of your phone system for outgoing telephony and especially for call forwarding. In our help section you will find instructions for numerous models. If in doubt, please contact the manufacturer of your phone system to assist you with the setup.