Hi @Viren Katariya ,
What is your product? Skype or Skype for Business?
Do you mean you want to transfer old phone number to Skype for Business?
Based on my knowledge, you can create a port order and transfer your phone number to Skype for Business. The following link shows detailed steps for your reference:
http://howtomicrosoftofficetutorials.blogspot.com/2017/08/transfer-phone-numbers-to-skype-for.html
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