The spell sending is a good option.
The spell Irriseni Mirror Sight lasts 1 minute per level rather than the one round Sending. It is also of a lower level. It allows two way conversation without sound rather than a single message each way. If both ends know lip reading (available as a linguistics skill), they they can talk to each other freely. If not, then they can write to each other while in view of the mirror. My character in Skulls and Shackles uses a network of spies that communicates via this spell.
/cevah