Other than the suggestions other people gave you I don't have any, but....I would like to call your attention on ways of getting him remember some instructions. I know there's a phase in which things don't "stick" in their memory, however, I would try the following:: if he enjoys being called and talking over the phone, try to find out things/activities that would resemble that need. For example, arrange for other people who care for him to call him and talk to him over the phone. Or other activities that would make answering the phone less atractive. Also, if you need to agree with him anyway on certain rules, help him with the "saving" of the instructions: this is, not only talk to him, use other senses and use experiencing. Ask him to repeat the instruction and hear his own voice saying it. Leave a piece of paper besides the phone in a way he could not help but notice it. Practice with him what he's going to do next time someone calls. Call him at home from your cell and repeat the excercise. Several times, and at different moments during the day. Make a game.
Thanks to your question I realize I have to think on that too with my mother.... Good luck.