While external access through a dynamic DNS provider is something that 3CX do not support I've found a source which claims it is technically possible: http://www.buchatech.com/2009/07/setup-3cx-pbx/ I've had a go at setting this up and internal calls between softphones (3CX softphone running on a windows box and 3CX softphone running on android) As a test I created 2 extensions: 10 and 11, connections were established. So far so good. I then switched the android device from wifi to the cellular network (3, UK) and tried to call extension 11 (the phone is set up to use extension 10). The phone registered but I was unable to make the call. Upon further investigation I found that extension 10 was reporting as registered but idle. When I was carrying out initial configuration the firewall test was completed successfully and I have the following ports forwarded to the 3CX box: IP_Telephony TCP 5480 5480 192.168.1.160 IP_Telephony TCP 5482 5482 192.168.1.160 IP_Telephony TCP 5483 5483 192.168.1.160 IP_Telephony TCP 5485 5485 192.168.1.160 IP_Telephony UDP 5480 5480 192.168.1.160 IP_Telephony UDP 5482 5482 192.168.1.160 IP_Telephony UDP 5483 5483 192.168.1.160 IP_Telephony UDP 5485 5485 192.168.1.160 IP_Telephony TCP 3400 : 3499 3400 : 3499 192.168.1.160 IP_Telephony TCP 5060 : 5100 5060 : 5100 192.168.1.160 IP_Telephony TCP 9000 : 9015 9000 : 9015 192.168.1.160 IP_Telephony UDP 3400 : 3499 3400 : 3499 192.168.1.160 IP_Telephony UDP 5060 : 5100 5060 : 5100 192.168.1.160 IP_Telephony UDP 9000 : 9015 9000 : 9015 192.168.1.160 Static public IP address is set to dansnowden.is-a-geek.com (as suggested in the page I found) and it was also set to be the "Public IP to specify in Contact and SDP" When I try to connect and dial I get the following in the 3CX log: When connected to internal wifi I got the following (When I made a call) The profile on the android device is et up to use dansnowden.is-a-geek.com as an external server, has the default STUN server of stun3.3cx.com. Does anyone have any suggestions?