If you are really concerned about your teens social interactions with talking to 'real people' then perhaps make sure there are plenty of opportunities where they need to practice these skills. Have friends over for dinner, or go out as a family to meet up with others as often as you can. Role modelling is simply the BEST way to show your teens how to interact.
If they are old enough to own a phone, they are old enough to volunteer! Find a charity, a market, a animal shelter, anything that would appreciate an extra pair of hands.
If your teen is old enough to work, then help in every way you can to get them a job.
If they play sport, awesome. If not, find out if there is any other regular activity that takes place somewhere and do everything you can to ensure you help them get there! As ironic as it sounds, there are now Gaming Cafes opening up which brings gamers together and they actually talk in person!
If your teen wants to go to the beach or city with friends, then let them!
It is not just a case of demanding your teen 'put the phone down'.