I have a Linksys PAP2 registered to a cloud 3CX server. I set the following parameters on the web admin page: Under SIP STUN Enable: yes STUN Test Enable: no In LINE 1 Line Enable: yes Nat Mapping Enable: yes NAT keep alive dest: yes Proxy: public ip of cloud 3cx Register: yes Make call without reg: no Ans call without reg: no Display name: 200 (ext. #) User ID: 200 password: xxxxxxx (password set in extension on 3cx) Use Auth ID: yes Auth ID: xxxxxxx (user id set in extension on 3cx) Under Audio configuration for Line 1 Preferred Codec: G711u Like I said it registers.. Here is the 3cx config: Phone is set to extension 200 Phone is provisioned as a Yealink T42S Provisioning method is Direct SIP (STUN - remote) MAC ADDRESS: set to Linksys PAP2 Select Interface: FQDN Local SIP Port of Phone: 5065 Codecs 1st is PCMU which is G711u Capture: When trying to call out: Request: INVITE sip:12105551212@publicipof3cx Status: 407 Proxy Authentication Required Request: ACK sip 12105551212@publicipof3cx Request: INVITE sip:12105551212@publicipof3cx Status: 100 Trying Status: 603 Decline Request: ACK sip 12105551212@publicipof3cx Decline Expanded:Shows Caller is Forbidden. Any assistance is greatly appreciated.