Some numbers you could dial for free, like the phone directory pages (if you stayed under 3 minutes). Some services were billed by the minute, like phone calls were.
Yes, in the sense that the French phone company handled all billing. So each month's phone bill showed all Minitel charges. This greatly simplified startups getting started.
At the same time, Minitel did not use the CompuServe/AOL model of centralized data hosting; each provider (whether large company or small startup) hosted its own data.[1] This gave startups flexibility by being able to quickly roll out new and updated data and offerings without having to wait for a third party.
[1] Part of the rationale for Minitel was for France to avoid dominance by American technology companies like IBM. Unfortunately for such a goal, Minitel providers generally used American technologies like Unix.
Also what was French voice phone service billed like back then? Did you have to pay per minute for non-local calls (or all calls, even)?
That's incorrect. There was a dialer on later versions of Minitel. And while you could technically set up a V23 BBS-like server behind a regular phone line to talk to Minitel terminals, the whole business model relied on the use of premium short numbers (361X) whose revenue was shared between France Telecom and service providers.
It was absolutely an App Store model and fortunes were made at that time.