The default configuration for all SIP trunk on Yeastar PBX is to check the circuit status with a SIP OPTIONS request. A reply back within 2000ms deems the trunk to be available & working. This is standard practice and is the default timer for Asterisk in general but there are situations where 2talk don't reply back within 2000ms and the trunk can be marked as unavailable for use.
If you are finding that the trunk is randomly not allowing outbound calls and the IP blacklist is clear then this could be the issue. Its easy to test, just remove the 'qualify' option on the trunk setting and retest.
We haven't seen any cases with any other NZ ITSPs where SIP OPTIONS requests are an issue. More info: https://www.voip-info.org/wiki/view/Asterisk+sip+qualify