One of the first things you should do is set office hours. Regardless of whether or not you have an actual office or if you give your tenants your cell phone number is irrelevant. One landlord I know has kept his landline telephone number for the sole purpose of using that for his tenants. He simply turns the ringer on the phone off outside of his preset office hours. He has an answering machine that allows him to screen those calls just in case they are an emergency, but otherwise he will wait until the next "business day" to get back to the tenant.
If you don't have a landline and don't feel like spending the extra money getting one, you can always get a Google Voice Number. You can sign up for a Google Voice account for free(!) and give this out to your residents. You can even set up a separate voicemail for that specific number and forward all business to that line. It will still ring on your cell phone, but it is a good way to separate your residents from your friends. You may want to include your personal number in the voicemail message in case of an emergency, but that is your choice. With a Google Voice Number, you can set times that your phone will ring and those that will send the call straight to voicemail.
Having tenants call you at all hours of the day and night is never an ideal situation, but thankfully there are other options that we have now. At the end of the day, you never know what a tenant will be like in regards to communication until they actually move in. Hopefully these ideas will help you find what works for you to keep you sane when it comes to answering your tenants and their concerns or complaints.