I wouldn't mind at all if I received such communication via any means I had provided to a business, but I'd be fairly upset if Random Startup B - whose service I demoed for a month and ultimately decided against - suddenly called me on a number I never gave them after I had already decided not to do any further business with them.
An email asking for a phone number would be perfectly acceptable. It would positively affect my opinion of the company and could go some distance towards gaining me back as a customer, but calling me on a number I never gave would be a pretty significant final nail in that coffin.