Extension Numbers: You can use extension numbers internally only, any VoIP user can call an extension number.
Phone Numbers: Phone numbers are real numbers, just like your landline phone number, anyone can call using any phone. Some countries support both voice and sms, see phone numbers
SMS Numbers: You can purchase a SMS Number to use with 2 way text messaging. Use your email account to send and receive sms messages. Your friend or college can send a text message to your SMS Number, you reply using your email account or Inbox account. You only need access to your email account or Inbox account to reply to any text message sent to your SMS Number.