Utilita Energy has bought Sheffield-based call centre specialist, bvocal.
All 325 bvocal staff – 225 at its Chesterfield site and a 100 at a Sheffield centre - will become part of the Utilita group of companies.
The firms have worked together since 2014.
The change comes into effect immediately and means Utilita now has more than 1,000 staff.
The firm claims to be one of Britain’s fastest growing independent energy suppliers.
Steve Parker, director of sales and marketing at Utilita Energy, commented: “bvocal has played an integral role in the growth of Utilita and has proved to be a highly successful sales channel.
“With this in mind, it made perfect sense to bring bvocal into Utilita in a move that will be mutually beneficial. As we continue to grow, bvocal will play a key role.”
Paul Stubbs, managing director at bvocal, added: “This is a fantastic business opportunity for both companies.
“Utilita has been bvocal’s only client for the past two years - this deal is great for our brilliant staff, who will now benefit from working for Britain’s fastest growing independent energy supplier.”
Utilita Energy’s customer numbers have shown strong growth in recent years.
The supplier expects to reach 500,000 customers in April – up from just over 200,000 18 months ago.
In January, bvocal opened a new office at Wards Exchange, in Ecclesall Road providing sales, customer services and compliance services.