The error message "SIP 923 / No DNS results / Service or option unavailable" is usually shown when:
- the server information is not provided correctly;
- the transport type is not selected as necessary;
- the outbound proxy provided is not needed or incorrect;
- the contact you are trying to dial is offline;
- the contact on the other side uses an unsupported by his PBX server audio codec (which causes him to seem to be offline).
Try pinging the domain/IP of the server and check your account configuration. If you are receiving DNS errors, you can also try to set the IP address of the VoIP server instead of the domain name.
On some servers (including Cisco Unified Communications Manager (CallManager)) this could be because the hostname used does not match the realm as configured on the server. For more information please see here.
With some of the vpn services the sip client can't communicate with the dns server(s).