Just a guess, as I don't use Skype, but have they ever worked reliably? If they did, and then it changed around April or May, it could be an update to the router they're behind that is causing the issue. My largest client is on vonage for business and ran it for over two years without a hiccup.
Once the router was updated for whatever the major security problem was that was revealed that month, they started dropping calls. That plus call quality that went to hell has ended up being my biggest headache of the summer. Put in a separate connection from the isp just for the phones, and used the modem/ router from comcast with all security disabled. Everything is now back to the way it was.
You could also try turning off the firewall on one Mac that was having issues, and seeing if that helps. If so, find out how to make exceptions for Skype in it.