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External Transfer on 3300 with SIP

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WGAIT:
Hi all,

We have a Mitel 3300 using a SIP supplied by a national UK provider. 

Everything works as it should, but we're unable to transfer calls to an external number.  For example, if someone calls our office, and we want to transfer that call to a staff member's home number (if they're working from home) the call goes through but there is no voice, on either end.

We used to have a SIP trunk provided by BT, who used a Proxy, and this external transfer worked fine.  We've made no other changes other than the SIP provider (who don't use a Proxy).  We're assuming it's Firewall/NAT related, but Wireshark shows both RTP streams are successful, and there's no lost packets, it's almost as though the two streams aren't being connected together as a complete call.

Wireshark shows the internal IP address of the Mitel handset making the external transfer on the OK200, which I gather is normal for a Mitel system, should this be set to the external IP address of the PBX somehow, or is this behaviour OK?

Does anyone have any ideas?  Thanks

dilkie:
are you using an MBG between the 3300 and the sip provider to provide trunking?

WGAIT:
Hi, thanks for the reply, there is no MBG.

ralph:
If your MBG is in the DMZ or another VLAN other than your voice VLAN then I'd venture to guess that the carrier's router doesn't have a route to it.
Have the carrier test by pinging the MBG from the router.

Ralph

dilkie:

--- Quote from: WGAIT on October 11, 2016, 04:57:25 PM ---Hi, thanks for the reply, there is no MBG.

--- End quote ---

Without an SBC of some sort, it's not going to work... that's what the MBG does for you, provides SBC functionality to anchor the signalling and media so NAT traversal to your carrier works correctly.

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