Avaya IP500, BT SIP Trunk and UTM - Calls both ways are fine but keeps line open after call

Question

Hi, 
 Customer has the following 
 UTM SG135w 
 Avaya IP500 
 BT Broadband (FTTP) 
 BT SIP Trunk 
 Its a new site, i.e. just finished building with internet going live. Installed the UTM to replace the provided BT Hub (router) absolutely fine, rock solid internet, phone calls working inbound and outbound. 
 Next day I got a phone call to say the Avaya handsets are saying "Waiting-for-line" or "Call rejected", no calls either way are working. They have the ability to make up to 3 calls. Spent the whole day there and simply baffled. By the time got there the calls were working but then after awhile the same issue would recur. Rebooting the UTM and/or Avaya (in different sequences at different times) made no change, same issue. However, if left long enough, between an hour or two, it would suddenly start working again. 
 What seems to cause the lockup is being on a single call for 15 minutes, after which if you hung up and tried to make another call then the same issue would happen. 
 The BT Engineer tried to help but told it was the UTM causing it but no real proof as to why so Im stuck on my own with this one. What was interesting though is the Avaya could register with BT SIP Service during one of these episodes and eventually he got it going again but Im wondering if it was good timing that the problem rectified itself. 
 During the last occurrence of this, I put the BT Hub back in place of the UTM and even that didn't resolve it, it was only until at least an hour passed that it sorted itself out. 
 On the UTM I had the telephone system able to go on ANY port to ANY destination, SIP Protocol off, QoS off, IPS off, Flood Protection off. When the system was working (with the UTM in place), every call was rock solid, no drops, no quality issues, it was only after you hung up from a long call then the problem starts. 
 One thing I have read, though not tried, is making the UDP timeout longer than the default. Wondering if anyone had any thoughts whether UTM is causing the SIP service provider to block the phone system or something crazy for X time. 
 I'm going to let them test the VOIP via a BT Hub to see if the issue happens but I have a horrid feeling its the UTM to blame somehow. 
 Just to add the issue starts even when the broadband usage is low, i.e. before anyone else is in the office using computers so I know its not a bandwidth issue. 
 Hope someone can shed some light on this and thanks in advance

SGICT · Accepted Answer

Thank you for your reply. 
 I left the BT Hub in place yesterday to allow the customer to try the phone system and within 30 minutes of them being in the office, they got "WAITING-FOR-LINE" message. 
 So its NOT the UTM. I'm relieved because I couldn't see what was wrong but glad I posted this so if anyone else comes across this then at least they can refer to it. I'm hoping to find out what was the cause and will post on here. I did read there is ASR setting on the Avaya IP500 that can cause this but not sure if thats the case.

SGICT · Answer

Hi Jaydeep, 
 Finally BT discovered the issue, the customer had moved premise so the old telephone system was still running and trying to register the same SIP Trunks! Obviously this would cause both the old and new telephone systems (in separate premises) to conflict with registration and cause this issue. BT should have decommissioned the old telephone system at the same time the new one was brought live. 
 Unbelievable!