Cell phones are probably the best way. Beyond that, calling cards...
Some team a few years back gave out calling cards for free.... I wonder- who did that? Also at
www.fatwallet.com there's occasionally offers for free calling cards- I browse there now and again to get free magazine subscriptions or pens and whatnot.
Also, if they have the internet and simply prefer phone usage I highly recommend Team speak- it's for the internet, and is voice chat just like a phone, but it's free.
Good luck keeping in touch with your FIRST friends, cause there ain't no friend quite like 'em.