The agreement should clearly define the duration of which you will be responding to an issue brought up by your client. Make sure to specify your business hours and if you will be providing services in addition to those business hours. If yes, then whether extra charges would be applicable for that or not.
Your Managed Services Agreement should clearly account for all the things you will be responsible for during your association with the clients. It should state what responsibilities will be taken up by you in cases of system, hardware or network failures or glitches.