All I can say about this topic is if I am going to be inconvenienced to one degree or another at the time of the call, the price will reflect as it should.
If I am in bed sleeping or about to, they will pay more.
If I am focused on a formulation of a product and cant really be interrupted or have people at the shop helping or bottling product which my direction and supervision is needed and I must leave, they pay more.
If I am working around the farm and I need to get things done and that was the only opportunity I had to handle things they pay more.
I guess the list goes on. Most every industry has off hours or emergency service charges I am sure. Once you pay your dues most adjust their response to this question. I have developed the philosophy over time that if they want me to respond to their problem on their terms and time frame that they will need to accept my fee on my terms or they can keep calling around.
Most people ( customers ) know and expect there to be an additional fee and generally the reply is, I don't care what the price is, I want you to come. So how do you pass that one up.
Just my .02 cents on this topic. That is how I have learned to roll over the years. If you feel it is just another call to you so be it. However I value and need my time as well to get other things done. So if you want to take me away from my scheduled plans that have priority at that particular time then you will have to accept my terms.
Many times no one else wants to take their call or respond so many times I was the only game in town so they can pay the fee or wait until the rate is what is considered a normal work day/hour rate. They have a choice as do I.
I guess in the end decision it is a matter of how much you value your skill set and time when you get such a call. Each time may catch you feeling a little differently. So if you feel generous or just had a long arduous day the outcome of your decision may vary for each of us. But at some point someone is going to pay.
I rest my case.