Does adding a subtype to a creature count as entering the battlefield?
No, attaching Call to Serve to a creature you control will not trigger Seraph Sanctuary's ability. A permanent triggers "enters the battlefield" abilities when it transitions from not being on the battlefield to being on the battlefield, whether it be from the resolution of a creature spell, from the creation of a creature token, or from some other instruction. However, Call to Serve just changes the type of a creature that is already on the battlefield.