We tried the Sony Babycall, but unlike the other moms who have replied, we had a lot of trouble with interference with that one and our (admittedly fairly old) cell phones. We couldn't go near the monitor when we were on either of our cell phones without getting awful, ear-piercing feedback. We switched to a digital monitor, the Graco iMonitor, and have had much better luck--no interference with our regular phones, cell phones, or wireless internet. We haven't tested the range much, but it works fine in our house, including between the basement and the second floor, where the baby is. The monitor comes either as a package with one transmitter and one receiver, or one transmitter and two receivers (alas, the opposite of the upgrade you'd want).
Oh, and in case you want to try one out: The Sony Babycall comes in a package that you have to cut open in order to get the monitor out, so it'd be tough to return if it doesn't work for you. But the Graco iMonitor (at least the one with the single transmitter and receiver) can be taken out of its packaging without cutting anything, and so would be easy to repackage and return if you tried it and nixed it.