should have/must have

English as a Second Language English Vocabulary, Grammar and Idioms
1

I tried to call him but the phone was busy for two hours.

[color=red]1-He must have left the phone off the hook.
[color=blue]1-He should have left the phone off the hook.

Both 1 and 2 are supposed to express an inference: If the phone was busy for two hours, then he has most probably left it off the hook.

Gratefully,
Navi.

2

#1 expresses an inference.

#2 is different; it means that he ought to have left the phone off the hook. In other words, there is some good reason, at least in the speaker’s mind, why leaving the phone off the hook would have been the correct thing to do.