A new smartphone app allows MBTA riders to instantly alert transit police to any suspicious activity they witness. The free, downloadable iPhone app, unveiled Monday, provides two options for contacting authorities. A button called “Report a Problem’’ on the app’s home screen allows riders to send text messages or photos to MBTA police. A separate “Call Transit Police’’ button allows users to speak directly to police. The app is part of the T’s “See Something, Say Something’’ program that encourages riders to report suspicious packages or any activity that seems out of line.