Consultants working for the Maine Public Utilities Commission are investigating CMP's meters and new billing system.
After a unanimous vote on Tuesday, MPUC commissioners agreed to expand the scope of the management audit.
Liberty Consulting will now review how CMP has handled consumer complaints and customer communication.
Customers have complained that CMP often was difficult to reach or was unable to help resolve their problems.
CMP has issued a statement regarding the situation: "We will provide Liberty Consulting with all of the information it needs for its audit of customer communications. We know that our customers look to us for reassurance when they have questions, and we have not delivered consistently on these expectations."
The MPUC said more than 24,000 CMP customers have filed complaints this year related to high bills and or unsatisfactory customer service.
"We'll continue to pursue this to the end and get to the bottom of it -- and we expect that to happen in the fall," MPUC Chairman said.
CMP said it has taken some steps to improve its customer service, including recruiting and training more people to reduce hold times and providing enhanced training for all its customer service staff.