Should I add in the contract that all communication hours (emails, answering questions) are billable hours? Is this standard?
You have to clearly define what constitutes billable communications.
It is not customary for any business to bill customers merely because they contacted the business. In fact, it would be fairly infuriating if you got charged every time you contacted any business. I, personally, would never want a client to feel they aren't free to contact me. That seems like a roadblock to lucrative client communications.
Now, with that being posted... long "meetings" over the phone or a messenger should be billable hours. If you have to take an hour or two out of your day to speak with a client about progress, changes, implementation, then yes you should be invoicing for that hour or two. For some clients, this is the only ting that will prevent them being a nuance. So invoice for meetings.. don't invoice for "communication" in general. Set a limit... anything longer than X minutes becomes billable.