We have simply requested our son, his new girlfriend and her 14 year old son not to use their mobiles while we are eating a meal or sitting chatting, but to check them, then answer later, unless it is really urgent. The same applies to computers in the sitting room when the whole family is together.
It didn't go down well to start of with, but they gave in albeit with ill grace and have since admitted that they enjoy not having to check their phones, Facebook etc. all the time.
When we visited them recently the same rules applied, so obviously they have found life less stressful without the constant text messaging, updating e-mail etc. At their place we were prepared to have to put up with their phones, after all their place, their rules, but it didn't' t arise.
We broached the subject originally by saying we hoped to enjoy their company while they were here and found it difficult to do so properly if they were always being distracted by their phones.