I know that sounds like a stupid question but I have heard it a lot over the years…  Particularly with cell phones that make it easy to just shoot your information to someone via email or Bluetooth.  There are many ways to find this seemingly obvious but easily forgotten bit of information but my personal favorite is dialing 1-800-444-4444.  This is some type of MCI auto attendant that reads back your calling number.  This works with individual lines such as a POTS (Plain Old Telephone Service) or cell phone.  If your phone is controlled by a corporate phone system your outgoing number may be a generic number for the entire organization so keep that in mind.