Does you ISP even allow you to own a modem with the voice ports. The problem is when you have voice ports there is a second special network running to your modem that has priority over other traffic for voice to work well. They don't want "smart" people from changing stuff to give their normal data this special priority.
Most ISP require you to use their rented equipment when you need voice.
Since ISP rip off for a voice line anyway I would consider just doing it yourself with one of the many VOIP solutions. Its been many years since so all I remember was they used to sell things like magicjack to do this. Make sure you get one that can do FAX if you need that ability.