What would you do if you had a client call you at 2 am?
A majority of people would "advise" not to answer, or keep the phone on silent or cut the call and message to call back during "office hours".
Put yourself in the place of the client. The client does know the time! There must be some kind of urgency/ reason for a call at this unearthly hour.
If you are a startup/ biz owner/ responsible employee/ a person with common sense; answer the damn call (calling the client after sunrise is as good as not calling).
In most cases, the client will have a small issue, but they will remember! They will renew the contract and you will definately get credit.