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New Member

SIP Error 403 Forbidden

Hey guys, quick question, We're an ISP. We have VoIP setup at a clients site thats failing to process phone calls. We have no idea why and we're completely stuck on this one. Basically what happens calls from any number on our VoIP network is unable to reach one of our clients. However that client is able to dial out to anyone. Setup is as follows:


They have a Cisco 3800 Series router thats connected to a voice gateway via an E1 controller card (ISDN)., also cisco. From the voice gateway it connects to a Cisco call manager system. The Gatway is responsible for routing calls to the PSTN for redunancy however during normal operation it routes calls to the 3800 router. Now the 3800 is supposed to process both  inbound and outbound phone calls from our network. However for some reason when an IP call is placed from our network to the client the calls fail with a SIP error 403 forbidden, however when trying to do a debug to see exactly where the call fails , either from the IP side (SIP) or failing on the V-gateway side (ISDN) , the router only shows us the IP side of the call with the 403 forbidden error. We turned on all ISDN debugs but the log never generates any errors from the ISDN side which makes it seem like its an IP error. No config changes were made and the setup last worked about a week ago. the logs on the gateway are blank meaning the gateway probably doesnt even get to see the call coming through (our initial throught was that the gateway may have been failing the calls).

Calls being made via the pstn work perfectly as i assume the calls going inbound would possibly go via the pstn if the ip side fail however calls from our network to the client fail.

EDIT: When looking at the 403 forbidden error, it also fails with a "cause=21" error, not sure if that means anything...

any ideas on what the problem could be?

Please let me know should you need further clarification..

New Member

SIP Error 403 Forbidden

Please post relevant gateway config. That should help!

VIP Super Bronze

SIP Error 403 Forbidden

Cause code of 21 refers to Toll fraud prevention which was introduced in 15.1(2)T IOS. A cisco gateway will reject any call from a source that is not in its trust list with cause code 21

To resolve this configure the ff on the receiving gateway

voice service voip

ip address trusted list

ipv4:xxxxx y.y.y.y (x.x.x.x=ip address of the source and y.y.y.y = mask)

Please refer to this doc for more detail

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New Member

Hey Ayodeji,

Hey Ayodeji,

Thanks for the hint on q850 CC 21. Am facing similar issue of getting SIP 403 Forbidden error with q850 CC 21 on Cisco Dialer.

Again thanks... 

I have posted the question on support forum. Please put a glance if time permits. URL:

Would appreciate your suggestions.


Ritesh Desai.

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