Latest figures show that Internet telephony services are rapidly growing in popularity.
Research group Point Topic reported on Wednesday that an estimated 11 million people were using a retail voice-over-IP service worldwide at the end of the first quarter of this year. This means that the number of retail VoIP users has doubled in the last nine months.
Japan leads the way, with around 4.5 million subscribers, followed by America with 2.1 million.
These figures also don't include software clients, such as Skype.
By mid-April, the SKype client had been downloaded 100 million times. There were 35 million registered users and 1.2 million people using SKypeOut -- a service that allows users to call fixed-line and mobile numbers for typically 0.014 Euros a minute.
Point Topic claims the number of actual Skype users is much lower than the total number of downloads, at 5.3 million users. This is based on dividing the number of minutes served by Skype by 7 (Point Topic's 'rule of thumb' estimate of the average time spend on the phone each day).
"This suggests that many people download the client and do not register, and many people who register never or rarely use Skype," said John Bosnell, senior analyst at Point Topic.
Point Topic's figures were based on a mixture of publically released subscriber numbers, and estimates for operators who don't release such information.