Whenever I have to make calls regarding some utility or service, I put the task into RTM (naturally!). Then while on the phone I create a note on the task detailing who I am speaking to and what they are telling me to do, when I save this it is automatically timestamped. If the problem isn't sorted out there and then, I update the task name (if necessary) and change the due date for whenever I want to follow it up. I then add additional notes for every call I make, repeating this process as many times as necessary, over time building up a detailed log of who I spoke to and when. This information has been incredibly useful a couple of times for being sure of what I've been told before and by whom. I often have more detailed records than they do! It has even helped me get compensated for a particularly shoddy example of customer service from my insurance company.

As well as this, I keep a note on the task updated with any general info I need for making the calls, for example customer ID, phone number to call, what numbers to press or extensions to dial.