Yealink SIP-T48G Administrator's Manual page 408

Administrator's Guide for SIP-T2_Series_T4_Series IP Phones
4. Configure parameters of SIP server 1 and SIP server 2 in the corresponding fields.
5. Click Confirm to accept the change.
To configure server redundancy for failover purpose via web user interface:
1. Click on Account->Register.
2. Select the desired account from the pull-down list of Account.
3. Configure registration parameters of the selected account in the corresponding
fields.
4. Configure parameters of the SIP server 1 or SIP server 2 in the corresponding fields.
You must set the port of SIP server to 0 for NAPTR, SRV and A queries.
5. Select DNS-NAPTR from the pull-down list of Transport.
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