Rightfoot,
I got my last 20' phone cord from a hole in the wall computer store.  It had a donut magnet choke built in near one of the ends, and it worked great.  Some people swear by those magnet chokes on modem lines.  I never did enough testing to have an opinion, other than the only really long phone cord I ever used for a computer had the choke, and it worked fine.  There are clip-on magnets that might work just as well.