I love that you're asking this. I would say something to them in person so that the message isn't distorted and is crystal-clear. Emails or texts can sometimes give off a different message. Ask how they would prefer to communicate. You don't mention the job, which is fine, but making sure that they can choose how to talk to others is preferable. Sometimes, I don't mind talking directly to others, but I still need that patience and time to get my words out. It's embarrassing still, mainly because it feels like stuttering often gets overlooked, but it'll be great in the long-term so they don't get stuck on a phone call they don't want.