Is there any way to set a start page depending on the user's dispositive?
IE. If the user enters in a PBI Service's Report from a Laptop/Desktop/etc, show him "Page A" by default , but if the user enters the same Report from Mobile App, show him another page by default ("Page B").
This is not possible today. You can suggest that in our UserVoice website here: https://ideas.powerbi.com/forums/265200-power-bi-ideas?category_id=105939.
A workaround for that would be to create phone layout to your report page, that is optimized for consumption in mobile devices. Once a report page has a phone layout setup, this is the layout your users will get when landing on that page from their mobile device: https://docs.microsoft.com/en-us/power-bi/desktop-create-phone-report.
Creating phone layout can be done from Power BI desktop or from the Web.