Yealink SIP-T2 SERIES Administrator's Manual page 685

Select DNS-NAPTR from the pull-down list of Transport.
5.
If you use outbound proxy servers, do the following:
6.
Select Enabled from the pull-down list of Enable Outbound Proxy Server.
1)
Configure parameters of outbound proxy server 1/2 in the corresponding fields.
2)
You must set the port of outbound proxy server to 0 for NAPTR, SRV and A queries.
Click Confirm to accept the change.
7.
Configuring Advanced Features
661