Hmm, spanielpups... just wondering if I could just add something here. I don't know if this will work, but it's worth a try.
My DD1 used to cry her eyes out everytime DH went away, and she would be like tremendously sad that she would miss daddy (it goes the same for anyone who is going away or even not seeing for a while... me, grandma etc).
Then I talked to her and told her that it was normal to miss a person, but that crying does not help to bring the person back earlier. Then I said that she could find time to make a card, and put all her love and how much she missed (whoever) into the card, so that when the person came back, she could give the person all that love, and that person would know how much he/she was missed. It seemed to help as it gave her something to do, and being told that it was normal to miss someone but that you could handle it differently seemed to help her a lot. So now, she's really good.
I think she was about 4 when I first explained that to her... and it's worked ever since.
Maybe for your son, you could explain the same thing - that calling you so many times would not reunite the two of you sooner, that it's normal to miss you... but that perhaps he could like make you a card or something, and put all his feelings in there to give to you when you came home, so that you could keep it with you and be reminded of how much he loves you everytime you looked at it... and maybe you could promise to keep it by your side when you go to work (I assume that you work) and that might help.
I don't know, because he's a boy, and my strategy worked with my girl... but it's worth a shot. The whole point really is just to let them know it's normal, and then to rechannel all the emotion and feelings into something constructive and reassuring... to let them know that you won't forget them while you're away...
hope that helps...