For any Call Connectivity issues, we recommend our customers to check the Signalling IPs/Ports are whitelisted, these can be found on the Service > PBX/Trunk tab the specific IPs we will be sending signalling from, so those IPs are only needed to be whitelisted.
What To Check
We use non-standard ports for SIP traffic. These are given on the Services > PBX (or Trunk) screen near the bottom of each entry along with the relevant IP.
Media IPs and Ports:
We use the media clusters that run on different IPs to the signalling IPs as they are used in the wider ranges we encourage our customer not to restrict any IPs and allow media from anywhere.
Also, with regards to the media ports we use wider range with 6000-65535 hence, advise our customer to allow these media ports
If the above steps do not reveal the issue, or you need to know what you can do to address the problem, please raise a case to us including any trace captures and the call url from the call log in the portal.