Speaking from experience in Crew Scheduling, it it comes to that much of an issue of basically 'he said-she said', anytime a scheduler calls to leave a message or notifies the pilot of an new/changed assignment, it should be recorded in either computer transactions or on the recorded phone line. Schedulers (at the airlines I have worked for, anyway) do not have the ability to fake a phone call or hide computer transactions. Any supervisor there can just check the time recorded in the transaction history, check the recording, and verify.

If you are absolutely sure you never got the call and want to be cleared, you can ask the recordings to be check. This happens all the time in scheduling. It's not like it can be a secret from anybody. They can even check which number was dialed by the scheduler. The recorded line cannot be turned on and off. It can serve as proof for anyone involved.

Even if the time the message was received is different tham the time called, it can be verified.