SIM Card doesn't work

I got an IPHONE 8  from my son.    He used T Mobile.   I  use AT&T.  When I   took the SIM  card out of  my IPHONE 6 and put it in the unlocked  IPHONE  8.  It said the SIM Card was not supported.    My contacts, pictures , apps are on the IPHONE 8 .  I can send and receive email but  I can not make calls. What do  I do.