Just to ensure there is not a handling problem. Normally the dialing should work that way:
- Click phone number in Zimbra.
- Wait until your phone rings. (Maybe message "Pickup phone to dial number #######" appears before ringing)
- Pickup the phone to dial the clicked number
- Message "Successfully dialed #######" appears
- Call should take place
Mabybe you just picked up the phone before it rang?