Draytek 2910G and Cisco phones

Hi All,<br/>
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I've a 2910G (3.2.1), thats doing some strange things with my Cisco phones.<br/>
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Short version: The Draytek seems to stop traffic from the Cisco phone, intermittantly, at random. Changing the source IP (at the phone) seems to fix.<br/>
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Setup is this:<br/>
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2910G provides WAN access to a remote office. The LAN there is used by both phones and PCs. Normally, the phones connect back to central office via SIP/RTP, via Public WAN Internet without any IPSEC tunnel.<br/>
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However, this Draytek does have an IPSEC tunnel setup, which secures PC traffic back to Main office, and a second tunnel for securing VoIP to the same SIP server (future planning, not really in use yet). I can change whether phone traffic goes via IPSEC by changing the destination SIP IP in the phone, if I need to.<br/>
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What I'm finding is that the phones will stop registering into the SIP server.<br/>
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Through various tests, I've found that if I change the IP of the phone, then it starts working again. Or if I change the IP of the destination SIP server, then again it works, for a few days. Or if I reboot the 2910G.<br/>
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other ciscos phones on the LAN will be working ok, at the same time.<br/>
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Id say it was a NATing issue, but, if I change the dest SIP server, to use the IPSEC tunnel instead then theres no NATing, just routing.<br/>
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Any ideas why the Draytek would do this ?<br/>
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Theres probably a total of about 8 phones, and maybe 8-10 PCs in that remote office.