Two modems for faxing

Hi, we want to set up internet faxing using XP’s built in fax program.
However it is very important that our incoming line is never interrupted therefore we need to have seperate incoming and outgoing lines.
To do this I assume we need two modem cards ( I guess one modem card and VOIP could be an option). I am wondering if XP’s built in service provides this or would we need to go another option?

Thanks for the help!