3CX Maintenance Package

Just wondering if anyone would mind sharing what they do and do not include in maintenance? If there's any issue is that covered, or just certain issues? Updates, new phones, etc. We have some on prem 3CX setups as well as a couple hosted. We are trying to come up with better standards to follow going forward. This question is posed mostly from a billing perspective​, as in what should we be charging for and what should be included in the sale. I understand actual prices would be region based so I'm not looking for that specific.