An active line will have 50 volts on the pair all the time. Drops to about 9 volts when the phone is picked up.
Was the line working before the building work started ?
If the line was shorted for a long time during the building work then the exchange may have "parked" the line. This is removing the 50 volt from the line in order to reduce current consumption at the exchange. Sometimes the line will automatically un-park, or you may have to ask BT to restore the line.
Or the line has been disconnected by OpenReach ( BT ) for some reason.
Was the line working before the building work started ?
If the line was shorted for a long time during the building work then the exchange may have "parked" the line. This is removing the 50 volt from the line in order to reduce current consumption at the exchange. Sometimes the line will automatically un-park, or you may have to ask BT to restore the line.
Or the line has been disconnected by OpenReach ( BT ) for some reason.