What is the difference between "in a call " & "on a call" ?

Answer 1

In a Call means your on the Phone -- for work (likely at work)

On a Call means your doing something physically -- for work

Like you mean be out of the office and at a Clients home.

On Call means your Employer, Employees, or whatever else related to work could call you at any moment and demand you do soemthing for work.

Answer 2

Generally if you are "On a call", your presence has been requested, such as by a client, and you are visiting them.

If you take this back before the telephone, "Making a call" or "Going calling" could be used when going to visit a person.