BT believes that the loss of many residents' phone lines in Church Hill has been caused by vandalism.
In a statement, the company said: "We believe three of our underground cables have been deliberately damaged.
"We urge anyone who saw anything suspicious to contact the police or Crimestoppers – there may be a reward.
"Around 300 customers (of any service provider) could be affected.
"It is regrettable when Openreach suffers any kind of damage to its network.
"Our priority is to restore services as quickly as we can, which is what we are trying to do."