Your experience is your experience. I don't doubt that what you have found works, works for you. Markets vary geographically, by industry, and by capital structure. Clients vary in expectations. Contractors vary in temperament. Projects vary in firmness and in scope. The only universal is that everyone should double their rates.
I did my first freelance gig in 1990. It involved Leroying. I've used fixed price. I've been billed out hourly to buy a job by a big firm that always did fixed price. I've chunked my time in half days. I've chunked it by the hour. I've eaten phone calls and charged for them. I've had clients who have more money than you or I will ever see and waitresses...and waitresses are great clients because they pay in cash and always have some in their pocket at every meeting. I've handed off a roll of drawings for $18,000 in cash from a safe placed in my other hand in the back of bar, drug deal style.
Some clients want high maintenance. Some are respectful of the fact that my time is money. Some are even charities and get some services for free. Businesses aren't charities.
All of which is to say is that reasons I wouldn't bill for phone calls or whatever were:
1. I was afraid of pissing clients off.
2. I didn't want to be that disciplined.
One day I realized that clients who get pissed off paying for my time aren't going to care what or how I bill. They simply don't want to pay and clients that don't want to pay aren't worth having because...well, they tend not to pay. So billing hourly and nickling and diming and requiring a healthy retainer against final invoice [along with the aforementioned higher rate] turns out to be a useful tool for screening clients. And providing a rate at the initial conversation is a useful screen for random leads.
So yeah, I'll bill 15 minutes for an email. It takes me at least that long to do it right. Charging aligns my interests (getting paid) with those of my clients (my doing the things they hired me to do right and not cutting corners). If it's not worth billing for then doing it isn't probably very important.
It is worth noting that I typically have a single point of contact. It's part of the industry landscape. Yesterday I was talking with a friend of mine from Jr. High. He's a chemist and the primary stay at home parent and does consulting on the side. In his industry royalties are money on the table: I learn something new every day.