I have 40 Nortel/Avaya 1120E phones that were donated to us (we're a 501(c)(3) nonprofit), and I have set up my TFTP provisioning server for those phones to deliver the appropriate SIP firmware and configuration files to set up for the connection to 3CX as the SIP proxy. The phones register, and outgoing and incoming calls work ok. After 30 minutes, however, I see the following in my log (I'm including the register message as well): Code: 27-Sep-2013 05:13:39.104 [CM504002]: Endpoint Extn:854: a contact is unregistered. Contact(s):  27-Sep-2013 04:43:37.214 [CM504001]: Endpoint Extn:854: new contact is registered. Contact(s): [sip:firstname.lastname@example.org:5060 / 854] After the phone is unregistered, I can still make outgoing calls on that extension, but incoming, as you may have figured, goes straight to voicemail. I can make the time shorter by shortening the Avaya's REG_REFRESH_INTERVAL parameter in the [DEVICE_CONFIG] file to below 1800 seconds, but making that parameter greater than 1800 seconds does not make the time longer; the default for REG_REFRESH_INTERVAL is 86400 seconds. Now the phones do re-register periodically, typically around 18-22 hours after unregistering, but the re-register doesn't appear to be correlated to much of anything else, like DHCP lease expiry or similar. Now, I know the Avaya 1100-series is not officially supported, but I have seen posts here and there on various forums that say people are using these phones, and they're working, but no one seems to post how they got them working (and I've done a lot of searching!). This is 3CX V11, SP4, build 28976. The Avaya firmware is the latest version, 4.3.18 (SP2, I believe). I've tried older SIP firmware on the phones; neither 2.2 nor 3.2 fix the issue. Oh, and the phones do not do this if I set them to point to a FreePBX Distro (Asterisk-based) server that I set up specifically to test this issue; the phones Just Work (TM) like one would expect, using the same settings as with 3CX, same firmware, and using SIP, not UNIstim (UNIstim, the native protocol, is supported on Asterisk, but not 3CX). So the basic phone configuration sent to the phones through TFTP seems to be OK (and I can post that if needed). I have six Cisco 7960's (SIP firmware 8.5.4) and four Cisco ATA-186's (latest 3.2 SIP firmware) working perfectly with the 3CX server, but these Avaya phones in theory should work. It seems like 3CX isn't handshaking with the phones the same way as FreePBX is; to the point that when registered to FreePBX the phones show the date and time, but not when registered to 3CX. Now, the 7960's registered to 3CX do show the date and time. Incoming and outgoing calls to and from the 7960's and the ATA-186's work perfectly, and DID's coming in from our RBS/CAS T1 via a Patton SN4960 are properly routed. So the basic system is working, except for the Avaya phones. In any case, if anyone has made an Avaya 1100-series phone (1120E specifically, running the 4.3 SIP firmware) work properly with 3CX, please post how you did it. And any advice on how to get these phones working with 3CX would be appreciated (getting other phones is not an option, so please don't suggest that......). Thanks in advance.