No voice when call forward to external number

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Feb 12th, 2010
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With the firewall turned on on sr520 calls ring to forwarded number but no voice in either direction.

Is there some ports to be opened to allow voice through.

Works fine with firewall off ?


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Steven DiStefano Sat, 02/13/2010 - 12:47
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What release are you running, how are you interconnecting to the PSTN (FXO, PRI, SIP Trunk)?

Are you able to make outgoing calls from internal extensions?

Are you able to receive calls from external numbers?

This will help us figure out how to help you.



andrewsymms Sat, 02/13/2010 - 12:59
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Yes can make and recieve calls from external numbers with firewall on sr520 off. Turn it on and makes connection but no voice


uc520 15.0(1)XA  SR520 12.4(20)T3

Connections from and to external lines are sip with a fall back ISDN if sip doesnt work.


Steven DiStefano Sat, 02/13/2010 - 13:04
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OK, so we need to understand the topology.   Is the SR520 in front of the UC520 with the WAN interconnect and acting as the FW, NAT & SIP ALG? 

In this case, the UC520 FW should be off as well as NAT disabled.

Or is the tolology different?

Steven DiStefano Sat, 02/13/2010 - 14:01
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I love the CCA topology view :-)   Nice!

So I dont think you have a problem.  The SIP INVITE will include the SDP message, and in it will be the negotiated of RTP ports, which the SIP ALG (on the SR520) needs to open.   The UC500 is not in play here, and shouldnt, as double NATs will do exactly what you say usually (no voice path).

SO you initial post is correct.  UC500 FW is off when you use a router/FW in front of it.

So sounds like you are working OK.  Correct?

But, if you are saying that the Call forwarding to a remote number (a mobile for example) is not working for SIP, then there may be something we can do there.

Who is the SIP Trunk Service Provider?

andrewsymms Sat, 02/13/2010 - 14:06
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I dont think so

Shouldnt the firewall be switched on on the sr520 as that is directly connected to the internet.

When I turn it on thats when there is no voice ?

The firewall and nat are off on the uc 500

Steven DiStefano Sat, 02/13/2010 - 14:44
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OK.   I understand better.

Check this out, this one shows ports to let throught the SR520

And this one describes your exact topology, but perhaps misses the ALG and ports

andrewsymms Sun, 02/14/2010 - 01:51
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Ive tried opening the ports as detailed in your previous link but still get no voice

Attached couple of screen shots that may shed some light on whats wrong




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