The most common methods are:
A Per-minute rate: clients are billed based on the time agents spend on the phone with their customers.
A Monthly retainer: Most professional answering services charge a fixed monthly fee that includes a certain amount of agent work time minutes. Additional minutes are billed on a per-minute basis. This is a good option for businesses as they are billed only for the time used.
Most professional call-answering services also charge a one-time setup fee to cover the cost of preparing and training agents to manage them.