If you are using a phone number that has ever been used before, it wouldn’t have been assigned a trial grant. Only the first use of a phone number could receive trial credits.
API trial credit for new accounts seem to have been deactivated recently, along with removal of mentions that were on the pricing page. There are more API developers than OpenAI can support, and the well-known entry-level services are now dirt cheap, so they may have decided that giving $5 (equivalent to thousands of interactions over three months) was a bit more trial than anyone needed.
I suspect the trial was just more tedium: accounts that never intended to pay a penny clogging up support, users getting upset that they never used all of $5 before it expired, SIM farmers selling “accounts” for use in prohibited countries on terms-violating sex talk API sites, lower standard of age verification, etc.
It is simple to add a billing method to an API account and pay the fractions of a penny it costs for the programmatic interface.