Network configurations that are common in office spaces can cause issues with the VoIP connection. If you are having issues, you can forward this info on to your IT team and they might be able to resolve the problem.
The ports that are used by zencastr are:
80 // HTTP
443 // SSL
19302 // Google STUN server
3478 // Twilio STUN/TURN server
Articles describing the problem in depth: