[wikibob]

The purpose of a work phone is to carry it during the specifically agreed upon period that you are actively on-call.

And you should be compensated specifically for that on call time. A standard is 1/3 of your on call time out of business hours is credited as PTO hours for 30 minute response.

Or 2/3 of your on call time out of business hours is credited as PTO hours for 5 minute response.