I saw this [Call hyphen Jim] last week on a sign advertising the services of a personal trainer.

Problem:
The hyphen is unnecessary.

Explanation:
Sign makers are notorious for putting apostrophes where they do not belong.

It seems that some sign makers are extending this bizarre habit to hyphens, too.

There is absolutely no reason to put a hyphen between the imperative verb “Call” and the person to be called (Jim).

Solution:
“Call Jim at …”