...
Zapier infrastructure runs on top of AWS services, so that if you want to limit the IP addresses accessing your PBX on the API port, you can enable a whitelist allowing requests going to your API port (same as PBX GUI - Admin GUI port) with these sources IPs (click on expand):
...
Even if we are working to find a better way to secure the Zapier interconnection, at the moment this is the only known and public way of securing your PBX when using Sangoma - FreePBX/PBXact integration.
More Details
You can find a better explanation and more details about Zapier used IPs here: https://docs.aws.amazon.com/general/latest/gr/aws-ip-ranges.html
...