Static/noise on phone line

I just switched to Xfinilty for phone/tv I already had internet.  I received self install kit. I hooked everything up first starting with cordless base plugged into gateway. This worked fine. I have an alarm as well as a couple of corded phones so need to use house wiring. I went outside into phone box and unplugged wire connecting house to phone service. Everything worked including setting off alarm and monitoring company recieved alarm. I then noticed back ground noise and humming while on phone. I unplugged alarm from that jack and noise is still there. I then used a splitter in gateway plugging cordless phone base into one jack and a wire to a house jack in other. Noise still there. I again unplugged alarm phone wire to eliminate alarm. Noise still there. If I unplug wire going to jack while on phone noise goes away. What could be problem with home wiring? Any thoughts as what to try will be appreciated.