MessageBird charges voice calls by the minute. We have a detailed overview of our pricing on the voice pricing page.
You can see how much has been billed per call by going to your Workspace name at the upper-right corner of your Dashboard → Financial → 'Billing log' tab.
Why am I billed more than it says on the site?
If a call is forwarded, you will be billed for both legs. Please consider that regarding call billing.