Communication with people with intellectual disability: What to do and what not to do...

Never deal with adult people with intellectual disabilities like with kids!
Treat them as all other peers; they usually love a good joke, as well as challenge (according to their capabilities).
Use clear and simplified language, try speaking slower, not louder (most of these people are not hard of hearing and speaking more loudly won’t make them understand you better...).
Most of these people can feel safer and more confident if they know ahead of time what will happen.
Keep boundaries and help these people to follow a good behavior (there is no need to tolerate or excuse bad behavior).
Ask them for their thoughts and opinions - and allow them to answer (don’t put words in their mouths).
Ask if you can help them - don't do for them what they can do themselves.
Expect a lot of questions - sometimes they need a good explanation.
Many people with Intellectual Disability prefer face to face and one to one communication.
In writing, use bigger text and bullet points, and keep writing at a minimum
On the phone (especailly) speak slowly and clearly, using easy to understand words.