Ask HN: Know any Calendly-as-a-Service providers?

I'm working on an application that will provide appointment scheduling to many B2B clients. Each of our (siloed) clients will provide unique scheduling options to their customers, meaning that each of our clients requires different appointment types and availability configuration options.

Basically, we need to provide a Calendly-like platform to each of our clients. Are there any existing providers that are built for this?

Ideally we'd be interfacing with a simple API that handles all the hard parts of scheduling. Bonus points if the provider has some pre-built elements that make building client-facing availability configuration really easy.