With the availability of the Enterprise Plan 5 which includes lot of premium services, including Dial In Conferencing, Microsoft proposes a tool to update your dial in conferencing number if you have enabled the Skype for Business PSTN Conferencing license

You can get the tool from http://www.microsoft.com/en-us/download/details.aspx?id=47729 (32 bits version)

Then deploy the package (very simple installation steps, just “next, next, next”)

Then start the tool and follow the wizard

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Select the user account for which you want to update the dial in conferencing phone number

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And update your existing meetings with the new phone number

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If no meeting appear, then you have nothing to update Smile