Setting up PBXs in particular is often complex and error-prone. If the call setup via your PBX fails, it is worth checking the settings for caller ID and SIP invite.
Please note that this topic requires advanced knowledge of VoIP and network technology. In this article, we assume that you have access to the PBX’s SIP traces. If in doubt, please contact your network technician.
How does a SIP Invite work?
When an IP call is set up, one side sends a “SIP invite” to invite another subscriber to the call. In addition to the “technical framework 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 number initiating the call is transmitted in the “SIP from address user part” field – “from-user-part” for short. If this is not the case, our infrastructure will reject the outgoing call attempt for security reasons.
A common problem: Incorrectly configured SIP invites
In everyday life, we regularly observe that systems are configured in such a way that the User Provided Number (UPN), i.e. the phone number to be displayed to the remote station, is transmitted in the From-User part instead of the SIP user name.
Please only use one of the following fields for the UPN:
P-Asserted-Identity
P-Preferred-Identity
From-Display
Remote Party ID
How it’s supposed to be
Here is a positive example with a correctly configured SIP invite. The key data of the call are as follows:
Number block: 030-80951 (000-999)
SIP user name: 00493080951
UPN: In the From-Display field (here the 03080951055)
Registrar: voip.easybell.de
And this is what the correct trace should look like:
From: "03080951055" <sip:00493080951@sip.www.easybell.com>;tag=xxxyyyzzz
SIP Display info: "03080951055"
SIP from address: sip:00493080951@sip.www.easybell.com
SIP from address User Part: 00493080951
SIP from address Host Part: sip.www.easybell.com
SIP from tag: xxxyyyzzzIf the data in the trace of your connections is transmitted as in this example, you can rule out the configuration of the caller ID as the cause.
If not, please check the PBX settings for outgoing telephony and especially for call forwarding. If in doubt, please contact the manufacturer of the telephone system to assist you with the setup.