phoenixstar: No, won't tie up your phone line at all. However, you should know that what you gain in convenience you lose in performance with phone-line networking. THese things are INCREDIBLY easy to set up. But they also have limitations on bandwidth, and won't be anywhere near as fast as a cabled solution.
Still, sometimes something like this, or wireless (which also isn't as fast), are your only solution. But if there's any way you can connect the computers with a physical cable, I'd opt for that route.