Their best and cheapest bet might be to just buy a few pre-pay phones. Swiss network operator sunrise has a store at ZRH airport and sells packages from CHF80 or so. These include a cell phone with basic functionality, a local SIM card and approx. CHF20 call credit. Additional credit can be added using scratch cards that come in CHF30, CHF50 or CHF100 denomination. The credit can also be topped up via credit card. The phones work internationally and your credit remains valid as long as you use your phone at least once every 12 months. So when the trip is over, as long as they send a text message from the US once per year, their accounts remain active.