Freeserve co-founder Ajaz Ahmed will take the role of chief executive and fellow founder Robert Wilmot will be chief technology officer.
Ahmed told ZDNet UK's sister site silicon.com he decided to take the post at Callserve because he thinks VoIP is "the next hot technology". Improved sound quality and the growing ubiquity of broadband are the factors he cites as contributing to VoIP's rise.
Five-year-old Callserve offers two retail internet telephony services: Callserve.com, which allows customers to call landline or mobile phones from a PC, and Phoneserve, a pre-paid calling card.
However, said Ahmed: "What we're doing today is not what we'll be doing tomorrow." Ahmed plans to roll out new services by the end of the year aimed at the European market. Currently Callserve does the majority of its business in the Middle East, Latin America and Africa.
Ahmed and Wilmot left Freeserve in 2001, after selling the company to Wanadoo for £1.6bn. Since then Ahmed has served as non-executive director for a number of tech companies, including Callserve (where's he been on the board for three years) and website designer Quba.
VoIP is becoming increasingly popular with enterprise IT directors who say the benefits of cost savings and extra features outweigh the risks of having a single point of failure.
Outlooks for the market vary. Juniper Research predicts the business VoIP market will reach $20bn in 2009, up from $4.54bn in 2004, while consultancy Analysys expects the European VoIP market will be worth €1.3bn by 2007.