When a second call comes in, the phone allows a test message to be sent to the caller.
The "standard" replies are not sufficient.
I want to be able to send my own custom text that says "Sorry, but I am on another call. I'll call you back as soon as I can. Thanks".
I sure as heck don't want to write this every single time a second call comes in!! Why can't I make and save a custom text message??
Or why don't you at least make this a standard message in the list of available text options? It's a little rude (I think) to text the caller "I'm busy, call back later" (the current "standard" reply). This message is not suitable for normal business practice, in my opinion.
Depending on the phone you have, you should be able to edit the quick responses via your Phone app. As noted earlier, if you could provide which device you're using, forum members could offer detailed instructions specific to your device.