2.In V73 version: Access to web GUI->Account->Advanced tab, set the “Accept SIP Trust Server Only” value to “Enabled”. Click confirm button to accept the changes. One point need to be attentive, please enable the SIP Trust parameter of corresponding accounts.
3.In V80 and v81 version: Access to Features->Generally Information, set the “Accept SIP Trust Server Only” value to “Enabled”. Click confirm button to accept the changes.
##For V73 version. The x of the parameter "account.x.sip_trust_ctrl " ranges from 1 to max accounts. For example, x ranges from 1 to 2 of T20.
##When you want to enable this sip trust control for account 1, fill 1 to “account.1.sip_trust_ctrl”.
##There is different syntax in latest V80 version. When you want to enable this sip trust control, please fill 1 to "sip.trust_ctrl"
##Enable or disable the phone to dial the IP address directly; 0-Disabled, 1-Enabled (default);
features.direct_ip_call_enable = 0