I have a very immature idea of a B2B startup, and I want to gather some feedback on it.
Assuming, that "future customers" are business people, what is the best way to communicate with them on early stages? Is it reasonable to appoint meetings without having any prototype? Or it would be better to write emails and make calls not to waste their time?
There are an increasing number of great resources that have been developed for exactly this purpose (amongst others). A few things that I think would get you off to a great start:
LinkedIn is an extremely powerful B2B networking tool in and of itself, but one of the great features are their Groups (organized by topic and interest). You will be able to get as much feedback as you wish to receive, and Groups can be a perfect place to run ideas by others in similar situations before you execute them. Here's just one example of a Group that seems to be a great fit: B2B Sales, Marketing, Social Media & Lead Generation
You wouldn't want someone wasting your time, so you need to offer something in return. Share something truly of value to them - reports, research, etc. - related to your target markets "pain point". Ask to chat with them 5 minutes to gather further information - be clear that its not a sales call. Be prepared to ask some good questions that validates the need, and if appropriate, ask for permission to follow up with them to share additional information that you gather from other conversations.
Take such information, create a MVP, then ask if they would be interested in validating the concept. At this juncture, many would ask for payment to participate if the validation process yields significant savings / value generation for the participant.