How and when in the conversation to do approach customers for a follow-up discussion or meeting?

Most appropriately when conversation is about to go away from desired direction. 

1. When you are unable to convince them over the phone.
2. When they are not100% clear

Good Day!!!

When you feel your conversation will not turn into a business deal, ask the customer for another business meeting.

After carefully listening to the customer in detail and knowing what the customers' intention to talk with you.