Authentication using Mobile number

I am in the process of developing web API(ASP.net) for a mobile app. The app wants to do authentication based on mobile number. My idea is

1. The user registers using his mobile number (An API will be called)
2. On registration the user will be sent an OTP.
3. Once the user confirms the OTP I will validate it with server.
4. If the OTP is correct I will send an HMAC key to the mobile user.
5. This HMAC key will be specific to that user only.
6. For accessing the web API the mobile APP will generate a signature using mobile number, HMAC key and timestamp.

Now, my concern is that there is web based Admin panel as well which uses same web APIs, how would i do authentication for it? To use Admin panel the user has username and password. I am looking for common authentication process. Please advise what will be the best way to keep authentication common and is my approach correct.