How can I be sure there's a connection between the telephone line and the power line? (Answer: they should not be connected.)
Many people can feel the 48V on a phone line pair. The big issue with 48V is that it is relatively safe and should not cause electrocution. 

There should be no direct electrical connection between the phone line and the power line. If there is you have a serious safety issue to get cleared up right away. 

The phone line itself operates as a current loop from one wire to the other. If one side of the phone line or the other is connected to GND or some other wiring circuit it will unbalance the phone pair and make the phone not work correctly.