...and I faced the exact same problem about 15 years ago. Up to that point, I never charged "phone time" with clients. Most of my work was programming and on-site support, training, etc. Billing was straight-forward and I considered talking on the phone a "courtesy".
But then I noticed some disconcerting trends: Hardware was becoming more reliable, requiring less hands-on-site time, and frequently I could talk clients through simple problems over the phone. I stared having weeks where it sure felt like I had worked 40+ hours, but was billing out a dozen or so. How could that be? Time spent on the phone!
Also, I had a few clients who figured out that it was much cheaper for them to spend a few hours working out issues over the phone that it was for me to come down and fix it myself. (usually in 1/10th the time) That certainly had to stop.
But what finally broke the camel's back was my first cell phone. My first plan gave me all of 20 work-day minutes per month, and you paid dearly after that. Clearly, I couldn't afford free chit-chat with that.
I wrote a very polite letter to my clients stating that due to changes in their needs, my business, technology and IT in general, that for the first time I would be charging for time on the phone. To make it more palatable, I even charge a slightly lower rate for time on the phone. The upside to them was that because of the cell phone, I was now far more accessible to them and was able to solve most "crisis" issues almost immediately no matter where I was.
Amazingly, absolutely nobody balked at this. They all knew it was reasonable, and were thrilled to have better, faster access because of the cell phone.
There's a certain flexibility I use in billing for "communications" time. I rarely charge for calls of only a minute or two. For the bigger clients that I bill out dozens of hours a month, I usually do not charge for "short" calls or messages".
But you do need to account for this time somehow. In a business sense, spending an hour writing an e-mail is no different than spending an hour writing code, researching a specific solution to a specific problem, or consulting on the phone. You are selling your time, and your clients need to respect all of it.
As for the case at hand, personally at this point I'd just start charging for the time (unless you've got a contract that would preclude doing so) If you're already charging for time on the phone, you should certainly charge for time responding to e-mail. Otherwise, write a simple 1 or 2 paragraph letter explaining the situation. They will respect this, or they won't. Either way, you can't afford to do what you've been doing. That's a lesson that it took me a good 15 years to learn a long time ago.
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