cracked:

Selective amnesia: A plot-convenient movie trope that appears more often than you think.

5 Movie Characters Who Suddenly Forgot Their Super Powers

I always wondered about the bit in T2 when he’s telling Sarah to ‘call John’.  I wondered if maybe if it was supposed to be that the T-1000 was more advanced and, in some ways, more human than the other Terminators.  The only reason for him to want her to do it rather than transform and call John himself is sadism – wanting her to be responsible for her sons death.

Or maybe it was just a mistake.