I nearly missed the call yesterday - I was walking across Exmoor to Combe Martin which is about 3 hours. I realised I'd walked about ten minutes into a zone of no phone coverage and was about to start back, changed my mind and just kept going forward and two minutes later phone rang :-)
Lesson : when in doubt WALK TOWARD THE BEER
(in this case the divine Exmoor Gold in The Focsle)