The mobile is switched on 24 hours a day while my son is "in theatre", not least because I am out of the UK for extended periods. It is a sign of paranoia and pessimism, an anxiety that something devastating can happen to someone really important.
The MoD has the number, part of a lengthy form filled in by everyone pre-combat. You hope the call will never come. But you know that the paranoia is justified, there really is someone, hundreds of someones, out to get him. They do not negotiate, they kill.
No one does ceremonial quite like the Brits and a wonderful example is the passing out of a new batch of recruits at the Royal Marines training camp in Devon. But, to be there on a sunny day, with a couple of hundred other proud parents, relatives and friends, is about more than the Royal Marines band, sons in their dress uniforms, and a mess picnic.
It is treated as the celebration of an end, but it is an end that is totally unknown. The gruelling training course – both mentally and physically – which offers a green beret as a prize would have been beyond me at any stage in my life. Its successful completion is a source of pride, personal for Sam, vicarious for me.
The members of the King's Squad are dismissed with the order: "Royal Marines to your duties, quick march." That is why the investment has been made, the end that is being planned.
Its consequences for those watching include an unending rollercoaster of contradictions. The Royal Marines are not back-office staff. They are a frontline cutting-edge fighting force. Sometimes, however, one almost wished that Sam's success, if it had to be in warfare, was as a computer expert. But they have been trained to operate as a team and there is some comfort in knowing how much it has been drummed into them to look after each other. Sometimes, you hope your own son will be selfish. You know that he is in the business of bravery and you would prefer that bravery is avoided.
You know that you support your son and all the others around him in doing as professionally as possible the job he is given. You cannot say you think that the politicians are wrong, or the whole adventure misguided. You cannot say you are amazed at how long it has taken them to say publicly what was so obvious, that the problem lies at least as much in Pakistan as in Afghanistan. Do such thoughts imply criticism and disloyalty?
It has required contacts not always available to everyone to find out even half of what has been going on to my son in the last few weeks – including a helmet-creasing bullet for him while close mates were being shot. That's too close for comfort. So much for keeping the mobile charged and switched on.