Get a few toys, something to build with, maybe, that are only brought out during the phone call times. Make it a novelty and they will be more interesting. My kids really like the bristle style building toys and the ladder building toys.
I don't work from home, but I do something similar while traveling or going to the doctor's office, car repair place, etc. This helps keep them entertained for longer periods of time, since they are able to play with toys they don't normally see. I put them away when we are done and they don't come back out until it's time to travel or go to the doc., whatever.
Have a few different ones on hand that can be rotated. You might bring out the mega blocks for a day or two, only during phone call time. Put them away and the next time it might be Lincoln Logs for a day or two. If you have about 4 sets of these building toys, you might get enough rotation out of them.
Oh, a great game from my son's 3 yr old party that I came up with. I bought a soft tether ball (comes on a rope-didn't need the pole-$14) and rigged it to our deck railing from a window cleaning pole. We taped up cardboard shipping boxes and the kids love to build the tower and use the ball as a wrecking crane. For your son to be able to do this on his own nearby, use a small ball and tape a string to it, use cereal and snack boxes that are smaller. Kids love to build and knock down. Have fun!