It's nice to know when someone is not available. But it's more useful to know when someone is. Roger responded to Joe's email with a question, and then proceeded to tell him the hours he would not be available to answer the phone.
Of course, letting someone know that you won't be able to answer the phone at certain times is far better than simply letting them try and fail to reach you during those hours. But speaking about unavailability is speaking about barriers. Informing of availability is speaking about openings. So instead of coming at it backwards and saying you can't take calls at certain times, come at it head on and say when you are likely to be available. It's a simple way to overcome a barrier to effective communication, and it's much warmer and fuzzier.