When IP restrictions are enabled in Zendesk, it shows this warning, "IP-based access restrictions can break third-party integrations. Allow all external IPs that need to access your account via the Zendesk API."
Whitelisting Ally's static IPs will resolve the issue and enable a smooth sync between Ally and Zendesk.
54.243.192.208
3.225.237.0
107.20.38.17
23.21.1.160
You can read more about our integration with Zendesk here.