A lot of DSL modems actually have routing functionality that interferes with your actual router. They tend to use a PPOE setup, which means you have to tell your modem to not do PPOE and use the settings that it had to tell your router it's a PPOE connection, complete with the login user/pass and external IP address the modem was getting. I know that sounds complex, so maybe just google on that subject with some keywords and you'll likely find the steps for it.
I've never seen a reason to switch to Voice over IP. The sound quality isn't as good, it's not really much cheaper, you may have to change your existing phone number, you don't have a phone if there's a blackout, and (for some VoIP services)