To elaborate on what Matha said, dial up services like
www.onesuite.com or
www.pingo.com charge about 3¢/min. to call a UK landline. You would just use up your minutes on the prepaid. IF you could find a cell w. free nites and weekends, that would be ideal. However, I think those "free" options are only available for post paid. Will you have a computer?
...mike
Make use of T-M's UMA/wifi free calling from any place in the world with access to wifi. I use an LG G6, wife an S7)
A/o Oct 20, 2013 no need for intl prepaid as T-Mobile U.S. includes voice roaming at 20¢/min (in and out)., unlimited text (in and out), and unlimited data in 140+ countries.
My Plan -[6 lines] U.S. T-Mobile unlimited minutes (incoming and outgoing), unlimited text, fast data on each line. that $145/mo. total! . (In U.S. no surcharge for calling a cell.) If a line exceeds 2G of data in a month, pay $10 more for that line. [That only happens a couple times/year.