I'm trying to get to the bottom of some strange SIP behaviour and hoped someone here might have an idea what's going on. 2 different customers have accounts with the same VOIP provider and they have several SIP accounts each. One of the customers (me) can easily have multiple devices registered to the same SIP account so they all ring on incoming calls, i.e. exactly is as it should be. But the other customer has a problem with multiple registrations. A single device can register ok and keeps working, but once a second device is simultaneously registered to the same SIP account, the first one may be dropped after a couple of minutes. If a third device is registered, then one of the others will definitely be dropped, within no more than 10 mins. While a device is registered, calls work perfectly. Initially I suspected that one of the SIP client devices was causing this behaviour, but that has now been de-registered and is no longer relevant, but the problem is still there. I am now suspecting the NAT router being used to connect this local network onto the Internet, but how possible is this? Can a NAT router be having an effect like this, making multiple registrations impossible? I must say I cannot see how, but it is one thing that is different between the 2 setups. What else could cause registrations to fail like this? Can anyone suggest any possible solutions to this? I have spoken to the VOIP provider, but they are of the opinion that multiple registrations don't work and are not supported. However, it is part of the SIP spec to support this functionality and it obviously DOES work as I don't have problems with up to 4 devices registered to the same account and they all work all of the time. However, it does mean they are reluctant to put any effort into troubleshooting it. Anyone got any ideas about why multiple registrations are failing in one instance, but not another? Ken G i l l e t t _/_/_/_/_/_/_/_/