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At my company we have 10 Mitel Teleworker phones (5312 model) which are connected to the PBX (3300 series) over a pair of dedicated ADSL lines (one at each site) all the phones are behind NAT on the consumer grade netgear router.

We've recently encountered a persistent issues of the Teleworker phones dropping calls and frequently dropping to a 'Please Wait...' screen upon which they eventually reconnect and calls continue until it starts again 5-10min later.

There doesn't seem to be any pattern to the times that this fault rears its ugly head.

Steps we have taken so far to try and remedy this issue include: changing the codec to the one which uses less bandwidth, having both ADSL lines checked by engineers (both are stable and provide 3000d/600u Kbps bandwidth) and checking that there's no other traffic saturating the lines.

Any help on this fault will be greatly appreciated! I'll buy anyone who finds the final fix a pizza/beer!

The one interesting thing we've found is that in the router firewall logs there's a lot of TCP and UDP traffic being directed to the PBX which is coming from the phones on random ports, and it frequently rotates to other ports which are also unlisted in any records.

This traffic is currently getting blocked, but once I allowed it (reckoning it might of been key presses or phone traffic) once I did this the issue got MUCH worse.

Any help will be greatly appreciated! I'll buy anyone who gives a solution to this problem a pizza/beer!

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What's the ping between the two sites? Mitel can't handle more than 40ms latency between phone and PBX without the call dropping. – Driftpeasant Dec 20 '11 at 14:42
Driftpeasant: is that still true with the teleworker setup? Typical pings are 60-70ms but I'm not sure they've increased significantly since, say, 6 months ago when it was working perfectly (Oh, hi, I work with Samuel!) – James Green Dec 20 '11 at 17:24
Clarify - you have TWO offices ? Where PBX located - at one of the two offices ? Are the calls inside the office with the PBX affected too ? – Sandman4 Dec 24 '11 at 19:59
Anyhow, my guess it's your ISP. – Sandman4 Dec 24 '11 at 20:04
Two offices, one PBX. Issues only occur at the remote office. – Samuel Jones Dec 25 '11 at 15:39
up vote 2 down vote accepted

The phones are likely using source port randomization. I wouldn't worry about that.

The problem might be that the consumer-grade NAT router is causing latency; I have seen those things experience problems trying to maintain a NAT table for as few as 5 users. If at all possible, try to use a VPN to traverse the ADSL links instead of NAT.

Mitel's phones are hypersensitive to latency. Their engineering guide has several recommendations related to reducing the number of hops between the phone and the PBX for this reason. You should have more than enough bandwidth though, as long as you are using G.729.

60-70ms is kind of bordering on what these phones can deal with. I'd suggest strongly that higher latency is being introduced by your consumer-grade routers failing, but it might also be that the ISP is slowing things up too (this is, of course, totally separate from whether they are providing enough bandwidth).

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