In GTD that is fixed through "contexts". E.g. you have a context "phone calls to make", and then next time you have time and are near a phone, you can start working on those tasks. Many todo apps have some sort of mechanism for that, or is this not what you mean since you seem to be familiar with GTD?

GTD has a lot of solutions for various types of todo list problems, but then again, you have to be a quite conscientious person to follow through on all of the details of it. At which point you probably have no problem with plain scribbles on some loose paper. My wife makes todo lists all the time (like, she's taking to someone and while she's talking she'll pull out some paper and write things down completely unrelated to the conversation, without skipping a beat) but she's very conscientious about actually doing those tasks, so she doesn't have a need for more complicated systems than small piles of papers with things that need to be done.

Whereas I make lists with breakdowns and several dimensions of categories and preferably according to a clearly defined, all-encompassing system, and it needs to work on all devices and include everything from what I need to buy for dinner tonight to my life goals, and then those notes become a job in themselves and I say fuck this and throw them all out and try the same thing again 6 months later. I'd much rather be like my wife.