Yahoo would not disclose the exact figures because of its policy of not sharing internal data, but Yahoo spokeswoman Monica Ma told vnunet.com that the rise was "substantial".
"During a two-day period from 16 to 17 August Yahoo Messenger experienced a substantial increase in usage of the Phone Out service," she said.
Phone Out allows Yahoo Messenger users to place low-cost calls using their PC to traditional phones and mobiles in more than 180 counties.
Yahoo Messenger also includes free PC-to-PC calling, instant messaging, text messaging and free voicemail.
"We do not blame anyone but ourselves," Skype spokesman Villu Arak wrote on a company blog.
Skype has now offered its premium users a free week of calls to make up for the outage.
"As a goodwill gesture to faithful Skype Pro, Skype Unlimited, SkypeIn or Skype Voicemail customers, we are adding an additional seven days to your current subscription free of charge," an email sent to premium users read.
"And even if you did not miss out on using Skype last week, you can still have a week free on Skype."
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