Users of two of the web's most popular instant messaging services will soon be able to talk to one another. Microsoft and Yahoo announced this week that their respective IM programs will be interoperable by mid-2006. The deal will benefit both companies, giving them almost as many users as the market leader AOL. That will give the firms greater opportunity to expand areas such as internet telephone services - called VoIP (voice over internet protocol). All major IM operators currently allow users to make voice calls between computers, but not to landlines or mobiles. "This is truly a turning point for the IM industry, and we believe our agreement with Microsoft will help usher in a new era of IP communications," said Terry Semel, chief executive at Yahoo. However, there is an irony to the deal given that the top IM services have resisted interoperability for years. According to Nielsen/NetRatings, 51.5 million people used AOL Messenger in September 2005, compared with 27.3 million for MSN Messenger and 21.9 million for Yahoo Messenger. Figures collated by research company Radicati show a similar picture, suggesting that even after Yahoo and Microsoft's IM software becomes interoperable, AOL will still lead the market with a 56% share.