Seeing the message 'No Service!' on the Yealink desk phone would indicate that the VoIP phone line hasn't registered successfully on the phone. 

Please contact our Support Helpdesk so that we can check your VoIP line credentials to make sure that these are all correct as the line should then register. 

If after checking the line credentials which if correct and the line still fails to register this could be due to a firewall issue is your network is blocking the VoIP traffic?  Try disabling your firewall to see if this then resolves the issue. 

Please also check your router settings to ensure that SIP ALG is disabled and that Quality of Service (QoS) is enabled on your router to prioritise VoIP traffic.