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...."

N510 software version >=240

In the web-interface go to: Settings - Telephony - Connections - DNS SRV Lookup

N510 software version < 240

In the web-interface go to: Settings - Telephony - Connections 

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