Hi network52,
First, please check if the SIP SLG is enabled.
Next, please check the RTP ports of both extension are opened and not
conflict between them and then that ports are forwarded to each internal IP phone.
For instance:
Ext 101 => Local SIP Port of Phone: 5064
Local RTP Audio Ports Start: 14000
Local RTP Audio Ports End: 14009
Public IP Address YY.YYY.YYY.YYY port forward to Ext-101 phone internal IP Address XXX.XXX.XXX.XXX
Ext 102 => Local SIP Port of Phone: 5065
Local RTP Audio Ports Start: 14010
Local RTP Audio Ports End: 14019
Public IP Address YY.YYY.YYY.YYY port forward to Ext-102 phone internal IP Address XXX.XXX.XXX.XXX
Also please make sure those ports are configured on cloud PBX under extension settings.
Note** The ports must be different on both extensions. Please find below screenshot.
Finally, please launch 3CX Management Console, go to Settings > Network> Firweall Checker> Run Firewall checker.
This will give you an exact list of what ports you would need to open on the 3CX cloud base system. If the test fails, you would need to open the required ports from your firewall.
Also, please visit the following links for more information.
Required ports: https://www.3cx.com/ports-used-3cx-phone-system-v14-v15/
Disable SIP ALG: https://www.3cx.com/docs/firewall-router-configuration-voip/#h.vbntni6x8gau
Configuration for remote extension:
https://www.3cx.com/docs/firewall-router-configuration-voip/#h.qibmgjnjn5ns
and https://www.3cx.com/3cxacademy/videos/basic/configuring-ip-phones/