Sounds to me like you could have a problem with interference on your telephone line. The electrical mains cables that run in your house could be run very close to your data cable (ie telephone line). Is this the case? don't forget could be in walls, under ground etc. Basically if you run 2 cables together that have different potential, ie mains and data, then the electromagnetic field generated by your electric cables build potential in your data cables. ie voltage is generated in your telephone line. this can act as a capacitor and the voltage can build up and up until the power supply is changed. ie you turn off the lights. This could have effects on your line