unlocked status

If a phone is locked into t mobile will it work with both post paid and pre paid plans.  I am trying to switch to a cheaper pre paid plan and only get wrong answers from the alleged t mobile reps.

i am trying to figure out if the phones are locked or unlocked to begin with...calling t mobile is not an option because they screw up so much they might get it wrong.

since I am on t mobile (post paid) now, is it safe to assume my phones are unlocked for pre paid

the phone models themselves are compatible...i haven’t had a current contract in 10 years, the phones were purchased for cash up front.

okay, main phone is unlocked, but back up phone can’t be unlocked until I use it for 40 days on the current phone number…..can I use the spare phone on t mobile pre pay even though i bought it through t mobile post pay?