[Jim S]

Wouldn't be an infraction if the T.O. hadn't been called. The substitution becomes effective when the substitute, in this case, either communicates the substitution, takes a position in the formation, or participates in the play. Simply being the 12th player on the field in not a foul yet.
He could go all the way across the field to communicate with the player he is replacing. Then the replaced player must leave. All the way back across the field. All with no foul as long as the replaced is off before the snap.
A substitute also becomes a player when he enters the huddle.