UCLA grad here as well...over 2 decades ago. How the hell did they get my number? It's not even listed. The only instituion that I had to give my cell phone number to is the California Medical Licensing Board. They insisted on my giving them my real cell phone number when I made an address change request in writing. In fact, they even called me on my cell phone just to make sure that I did provide them with a real cell phone number before they would even change my mailing address. UCLA could have only gotten my cell phone number from them, since I didn't even had a cell phone 20+ years ago.
It could be that you had inherited a UCLA graduate's number. Just tell them that you are not a UCLA Alumni, and have no interests in donating to UCLA. Good luck!