From an MEC update, 10-15-2015:

Because reserves are required to check their schedule on a day off between 1800 and 2359 (20-K-7 as modified by the JIT), this must be accounted for as company-required duty. A one minute duty period will be recorded at midnight of the first day on reserve (interim process pending programming changes that move this time to 1800). Following this one minute of duty, a minimum of 10 hours of rest free from duty is required.
Therefore, a reserve coming off days off is not legal to report for a trip or begin a SC/FSB until 1000 on his first day of reserve unless the crew desk achieves two-way contact (see Important Note below) or the pilot aggressively picks up an assignment.

IMPORTANT NOTE: If the crew desk calls you on your last day off and achieves positive contact (two-way telephone conversation) they can give you an early SC or trip that departs as early as 0600. If such positive contact is achieved, there is no requirement to enter the 1 minute of duty at midnight but contact must be made at least 10 hours prior to the report or SC/FSB start time.

Also, leaving a message is not considered legal contact. So, if they leave a message at any time, that is not adequate. I've had them recently leave a message prior to the legal window, on my day off, and then they think I am on short call the next day. That is not legal notification.

Reserves, no there is no way to acknowledge an assignment. That is supposed to be there, but alas, another thing not implemented.

And no, you are not being paranoid.