Protection One Inc. wants to build its business by doing business with one of the country’s biggest communications businesses.

Their collective target: more businesses.

Protection One, a Lawrence-based provider of monitored security services, announced Wednesday that it had expanded an existing agreement with BellSouth Corp. — adding plans for marketing its services to businesses throughout BellSouth’s nine-state region.

The two companies already have been teaming up for nearly four years to offers services to residential customers under the name BellSouth Security Systems.

Wednesday’s announcement gives Protection One a chance to call on a whole new set of potential customers.

“We have a relationship with BellSouth, which has access to and relationships with millions of business owners or potential business owners in their area,” said Robin Lampe, a Protection One spokeswoman. “This is a real opportunity for us to reach out to those business owners with our expanded and growing security portfolio.”

Terms of the agreement were not disclosed, but BellSouth said it was drawn to the ability to continue building broadband business options through the addition of security products. Among them: closed-circuit television systems, remote-video services and access-control systems.

“It’s part of our commitment to exceed customer needs in today’s security-minded marketplace,” said David Scobey, president of BellSouth Small Business Services.

Protection One shares surged on the news, closing Wednesday at $20.50, up $2.75, or 15.5 percent, in over-the-counter trading. Quadrangle Group, which manages about $2.5 billion in assets through an equity fund focused on media and communications companies, and another group that invests in financially troubled companies, owns about 97 percent of Protection One shares, which underwent a reverse stock split earlier this month after Quadrangle agreed to restructure about $120 million in Protection One debt.

Shares of BellSouth closed at $25.63, up 22 cents, on the New York Stock Exchange.

Protection One has about 70 employees working out of its corporate headquarters in the I-70 Business Center, 1035 N. Third St. in North Lawrence. The company has more than 1 million residential and business customers in the United States.