Theoretically, you are supposed to be ready and available at base while on reserve. In reality, many crew members live in other cities and "take their chances" with being called out/utilized. If you are called for a trip and cannot commute to your base in time to make the assignment, you receive a "missed trip" and it goes on your record. If the problem persists, you will be reprimanded and eventually terminated (if the problem is really bad).
Reserve assignments are usually issued via taped phone message by 1900 the night before the trip. If there are no trips to fly, crew schedule will usually place you on "ready reserve," which means scheduling can call you for an assignment with very little notice if need be. I think most commuters who serve reserve share an apartment or hotel with other reserves while on duty.
Take care,
Art Tang
IMA