A warning message pop-up when attempting to call an offline BYOD user, similar to the warning presented when texting an offline user, with verbiage like "Contact offline - User may not be on duty".
There is no mechanism in place to prevent BYOD users from receiving calls when they are offline, but this type of warning message would help deter users from placing those calls.
Idea sourced from customer The Valley Hospital, see SF case 00459525 for reference. Thanks!