Calls and texts to the UK are free on AT&T, Sprint, Boost, and Virgin for a limited time following terror attack
Until the end of May 24, British time, AT&T and Sprint will either waive or credit charges for calls placed and texts sent from America to Britain. Virgin and Boost, Sprint's pre-paid virtual operators, also offer free calls and texts between the US and the UK.
T-Mobile and Verizon are yet to make any moves or statements, though it's highly likely that they eventually will. Over at the UK, local carrier EE will let everyone charge their phone or use an available one to call someone.
source: AT&T , Sprint via Android Authority