We use a spreadsheet to track our questions and the client's response, and we put a "Response needed by" date next to each one. If they don't fill in the response on time, the schedule gets bumped. You can use a shared document through Google, or just ask that they email back your responses. Each week we add more questions and send an updated spreadsheet. If they never respond, then you have ground to stand on when they ask "Why didn't you finish by the due date?".
We take the time to explain how the spreadsheet tracking system works up front, and we typically get a positive response. When we get angry faces and grimaces, we usually know it may not be a client we want to be working with anyways...
We also send the client a monthly progress update with the current status of the schedule, what has changed in scope, and where we are at on budget. These monthly updates are again a way to document progress, and it also serves to prevent surprises when they learn of changes to the scope/schedule/fee. I attached a template example if you want to look at it. Even if they don't answer the phone, these progress updates are legal documentation of your efforts to keep the project moving. A documented list of times that you left a voicemail doesn't hold up as much in negotiations as a list of letters sent to the client. Good clients will find these progress updates helpful, but bad clients will find this annoying because they won't be able to screw you over like they have previous engineers.
I hope this is useful~
