Are there instances where you DON'T share an agenda with a customer prior to a call?

When they requested the call or if it's some sort of tactical check-in where you want them to feel comfortable raising any issues

It depends on what you mean by an agenda. While it is always important to provide a valid business reason for the meeting so everyone is on the same page, a detailed agenda isn't necessary in most cases. Understanding your customer's expectations is key. Have they, or do they usually ask for an agenda? If they do, then provide one. If they don't, the reason for the meeting is probably sufficient.

Only when they have requested for a catch up. I usually get the agenda from them. Otherwise we know we are pretty much calling each other to update on something that we promised on.