Your biggest concern should be whether the employee is subjected to the company's control to the extent that he or she cannot do other things during the on-call time. If so, then you will most likely required to pay for all of the employee's time when the employee is engaged in that manner. This is a fact-specific issue and while there are general rules, there are many nuances that make the difference.
If you run a business in California, you should have an on-going relationship with a business or employment attorney to whom you can ask such questions, as well as one who can review your policies from time to time to make sure you are complying with all laws.
twitter.com/MikaSpencer *** All legal actions have time limits, called statutes of limitation. If you miss the deadline for filing your claim, you will lose the opportunity to pursue your case. Please consult with an experienced employment attorney as soon as possible to better preserve your rights. *** Marilynn Mika Spencer provides information on Avvo as a service to the public, primarily when general information may be of assistance. Avvo is not an appropriate forum for an in-depth response or a detailed analysis. These comments are for information only and should not be considered legal advice. Legal advice must pertain to specific, detailed facts. No attorney-client relationship is created based on this information exchange. *** Marilynn Mika Spencer is licensed to practice law before all state and federal courts in California, and can appear before administrative agencies throughout the country. She is eligible to represent clients in other states on a pro hac vice basis. ***