Onboarding: Make it as easy and smooth as possible for clients to get started after signinup. Show them exactly where and how to start.
Documentation & FAQ: Create tons of it. If a client has a question, thy should be able to resolve it through your documentation for the most part. Don't let little simple questions to come to you EVERY time.
Setup a Support Ticket system and only answer via emails/support ticket for questions that cannot be resolved via your documentation. If a client is not aware of documentation, point them to it before answering the same question again and again.
Get a decent smartphone and answer the tickets/email through that. You could even do it sitting at your desk or during lunch break
If you absolutely need to schedule phone calls, schedule them during lunch break and find a relatively quiet place where you can talk. If not quiet enough, tell the client that you are travelling and they may hear background noise. As long as it is not a screeching train, clients won't mind specially if you already told them.
Hustle. Do whatever it takes to get the first few clients except illegal activity of course. You may have to cross a few lines at work (lying about lunch plans etc) but I personally think those are reasonable to do.