The Be’er Moshe, Rav Moshe Stern (a”h) states that it is permissible to set the automated system on one’s phone to ring on Shabat morning to wake him up. Since the system is entirely automated, and no one is doing a forbidden act on Shabat on the person’s behalf, it is permissible, and we do not forbid this out of concern that people might think that he is using the phone. However, the Be’er Moshe adds that one should put the phone at a distance from one’s bed so that he does not inadvertently answer the ring or press buttons when the alarm sounds in the morning.
This applies only to an automated alarm system. One may not ask someone – even a non-Jew – to call him on Shabat morning to wake him up, as he will then be having melachah (forbidden activity) performed on his behalf on Shabat. Therefore, if someone is staying in a hotel over Shabat and wants to request a wakeup call, he must determine what kind of system the hotel uses. If the system is automated, and the call is made by a computerized system, then one may request a wakeup call, but if someone at the desk makes the wakeup call manually, then one may not request that he be woken on Shabat.