Please check that you have an internet connection and that your phone is registered/getting an IP address.
If SIP-ALG is enabled on your router, please try disabling then reboot your phone.
Please make sure that your phone has the latest firmware.
If manually configured, please ensure that all details are correct.
If using auto-provisioning, please make sure the provisioning URL is correct.
If none of the above works, factory-reset the phone and re-provision.