Up to 800 homes and business in Berwick were affected
Hundreds of people were left without their landline phone service for nearly 48 hours after thieves stole 1km (0.6 mile) of copper cabling.
Sussex Police said the 6in to 8in cable was cut and removed from the area near Drusilla's roundabout in Alfriston in the early hours of Wednesday.
BT said telephone services to up 800 homes and businesses in Berwick were restored on Thursday evening.
The estimated cost of repairing and replacing the cable was £45,000.
Officers believe thieves posing as workers wearing uniforms or high-visibility jackets used a vehicle to pull the cable out of the ground and take it away.
Pc Thomas Stallard said: "Manholes running along the side of the road were used to access the cables and it appears the grass over some of the manholes was dug up to reach the sealed covers.
"The cable is 6in to 8in in circumference and is very heavy."
He said cables would have been live and that was "extremely dangerous".
A spokesman for BT said: "All work has now been completed. Cabling work finished last night [Thursday].
"There are still a few outstanding faults but overall the work has been completed.
"When something like this happens we'll often offer to divert calls to people's mobile phones."
Sussex Police have urged anyone with information about the theft which happened between 0100 and 0200 GMT to contact them.