As per usual, this is a network issue. You can use Microsoft Network Monitor with port and IP filters to check connexions.
Normally, the issue is the following :
The client is making an outbound call, the call has 3 ways to process and will try the ways as follow :
- Internet (Edge) – The call ask the EDGE if someone with the following sip is connected trough him.
- PSTN – Your PSTN Gateway
- Voicemail – Your UM Exchange server
For any network reason (Switch, Router,Firewall,…), the client can reach the edge, but the edge can’t reach the client. This is causing several beeps before the call really begin.
Hope this helps!