Question: I am trying to get a Linksys SPA942 registered as an external extension. What settings on the phone do I need to change? appreciate any help. Will update tomorrow Answer: Step 1: Make sure your firewall settings on your system/router are open. My settings for my router: Firewall: SPI (Stateful Packet Inspection) - Disabled Port forwarding: (Found these ports in 3CX wiki) SIP TCP/5060-5060 -> Local IP of 3CX machine SIP TCP/5090-5090 -> Local IP of 3CX machine SIP UDP/5060-5060 -> Local IP of 3CX machine RTP UDP/9000-9049 -> Local IP of 3CX machine Step 2: The SPAXXX phone From a factory reset (DHCP enabled) these are the changes I made. SIP Tab: SIP Parameters: SIP TCP Port Max: 5090 (changed from 5080) RTP Parameters: RTP Packet Size: 0.020 (Was recommended in 3CX video) Symmetric RTP: Yes (This was the setting that made this a living nightmare for me) NAT Support Parameters: STUN Enable: Yes STUN Server: stun.3cx.com Ext 1 Tab: NAT Settings: NAT Mapping Enable: Yes NAT keep Alive Enable: Yes Proxy and Registration: Proxy: 99.14.10.xxx (Public IP of 3CX machine) Register Expires: 300 Subscriber Information: Display Name: External Extension User ID: 19 Password: (password setup for this extension) Special Thanks! To: KerryG - Thank you for your reply, this isn't the first post you've helped me with. leejor - Thanks for having the patience and helping me step by step in this process. You are very active on the forums and I am sure we all appreciate your time and knowledge!