When you engaged the company, they should have sent you a contractual letter. This in turn ought to have outlined the scale of charges for each type (grade) of chargeable resources. Refer to that in the first instance.
A visit to a legal practice is a real eye-opener; these businesses have little clocks (or maybe some more advanced technology now) in place at the top pof the desk that the individuals switch over between client jobs that tracks to every 6 minutes (0.1hr). It means when they pick up the phone, they can start the appropriate charging clock.