Hearing aids require a device - usually called a streamer. This is typically on a lanyard and that's what the phone pairs to and it also has the mic. Streamers are used for all kinds of assisted hearing (TV, speaker's with a lapel mic,etc.). Check with your user to ensure that they have a streamer and take it from there.
Here are a couple of likes as examples.
http://www.oticon.global/solutions/accessories/connectline/
https://www.youtube.com/watch?v=txOihJPheFY
