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.
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source: AT&T , Sprint via Android Authority
Things that are NOT allowed: