Introduction

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).
Background is RFC 3263: "...If the TARGET was not a numeric IP address, but a port is present in the URI, the client performs an A or AAAA record lookup of the domain name...."

In the web-interface go to: Settings - VoIP Providers - Edit 

Make sure that the port numbers are empty, see the picture.

• Proxy server port must be empty.
• Registration server port must be empty.
• Outbound proxy port must be empty.

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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.

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