Checking if a sippeer is online


In Asterisk, i have some extensions where I dont dial right away, instead I answer the call, play something to the user, and then dial.

What I want to do, is to check if the user has started his softphone, and if not, dial another number.

Right now, I dont know if the user has started before I dial his number, is there a way to check if his softphone is registered before playing the messages?