Issue regarding phone number

Question

I have problem while entering my phone number. First there was a phone number in my Microsoft account. I thought that I should remove it and should enter another phone number. So I first remove my old phone number. Later I thought that that old phone number was OKAY. So I tried to enter my that old phone number again, but I was not taking that number. It is writting an ERROR that this alias cannot be added to your account, please take a different alias.What can I do now?. I don't have any another phone number. Please help!