Do not use the preconfigured SIP server port numbers (5060), if you want to make use of SIP server redundancy (“Locating SIP server with DNS according to RFC3263).
In the web-interface go to: Settings - VoIP Providers - Edit
Make sure that the port numbers are empty, see the picture.
The N720 changes to secondary server when the primary does not answer after a time or it returns an Error 503.
If you would need that also on an Error 500, the device changes to the secondary server, you need to change this in the configuration.
If you have no provisioning possibility, you can use the patch to enable/disable the above parameter.