Respond-aparato is awfully clunky, Chainy. On hearing that I would imagine it refers to something more elaborate than a simple domestic answering machine.

Are we now to refer to a computer as a komput-aparato or a ruler as a lini-aparato?

If you look at the contrast between flugilo and flug-aparato, trikilo and trik-maŝino, mezurilo and mezur-aparato, I think it is obvious that -ilo is used to refer to the most commonplace realisation of doing something, leaving maŝino and aparato to other more complex manifestations.

This argument could be used to justify limiting respondilo to 'reply form', but I am not convinced.

Question arising. What is a televidilo? What is a televido-aparato?

sudanglo:Respond-aparato is awfully clunky

In comparison with 'respondilo', then yes, I suppose it is, but it's not wrong. I think that most people would opt for the form ending in -ilo just because it rolls off the tongue much easier.

-il- can be used for both small/simple things (ŝlosilo), as well as bigger, more complex items (aviadilo). In the case of 'aparato', then you're probably right that this is generally used for the bigger and/or more complex items, but then I suppose it's always a bit tricky to pinpoint the exact point at which it is perhaps not the best choice.

In the case of 'answering machine' ReVo accepts both forms 'respondilo' and 'respondaparato', and I don't see a problem with that. After all, if someone were to ask "Kio estas telefona respondilo?", it seems pretty normal to reply "Ĝi estas aparato, kiu aŭtomate respondas al telefonaj alvokoj".

An answering machine is indeed a combination of various parts to create a machine for a specific function, which fits the definition of 'aparato'. It's not like 'kudrilo', which is a simple one-piece instrument. You certainly couldn't call that an 'aparato'!

sudanglo:Question arising. What is a televidilo? What is a televido-aparato?

They can both be used to mean exactly the same thing. Just that the 'aparato' version is quite a mouthful!

Televidilo certe estas aparato!

Although Esperanto is not English, I think the case of "aparato" and "apparatus" is more or less equivalent.

"Apparatus" isn't a foreign word to us. We all know it, and have probably used it at some point or another. It's a regular English word.

But it isn't a "go-to" word. It's the sort of word you only pull out if you want to call attention to the complexity of a device. If the complexity doesn't matter, you generally choose a different word.

I would say that while an "ilo" can certainly be an "aparato" (and vice versa), unless the fact that there's a lot of component parts is important to the context, "ilo" is the better descriptor every time.

A television is definitely an apparatus. But when we're talking about them, it's usually not important to the context that there's a tuner in there AND some circuit boards with chips, and a display device, some speakers, and so on and so forth. Yes, those things are all there, but we still tend to think of a television as a single unitary thing. So, "televidilo". "Televidaparato" might be more appropriate for discussions about fixing televisions, or building them, or whathaveyou, where the existence of component parts is important to the discussion. Even then, possibly not.

Anyway, short form: I'd expect "aparato" to only come into play in fairly narrow contexts. I think of "ilo" as the "go-to" for device-related terms.

How many people still have answering machines?

What's a good term for "Voice Mail?" Would respondilo suffice or would you need something like voĉpoŝto?

"respondilo" works equally well whether it's electronic voicemail or an actual answering machine, in my mind. "respond-aparato" wouldn't work with voicemail.

RiotNrrd pretty much hit it on the head, I think.

In a nutshell, I think we should remember that the key goal of Esperanto is to foster better understanding between cultures. That means avoiding clunky compound constructions when a simpler alternative is available. Now for technical discussions where extra precision in important, this principle often takes a back seat, but in general it is a good rule of thumb. I think we're having trouble seeing the forest for the trees.

Respondilo in most contexts is perfectly understandable. If there is a doubt that the interlocutor/reader will correctly conceptualise "response-tool", then "telefona respondilo" most succinctly and clearly qualifies the noun.