Wouldn't there be a similar question for the Sacrament of Marriage?  Why can't couples be married over the phone?  Or on a Zoom session? etc, etc

I think this is a slippery slope.

Ok, then what about a deaf person?  Can't hear the priest's voice, so can't be absolved?

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No, the difference has to do with being in someone's proximity or otherwise. The soldiers who are too far away to hear the absolution are not morally present to the priest. That's why they can't receive absolution from him. Whereas a deaf person in close proximity to someone is still in close proximity even though he can't hear him.

I don't see why confessions couldn't be done by video chat.  Perhaps there is a slight risk of hacking. I'm going to ask a priest.

There is much less of a risk, than the risk that someone outside the confessional (perhaps someone with very acute hearing) could hear one's confession.

Here's what Jone's Moral Theology has to say about the matter of confession by phone:

It seems to me that Sacraments & blessings always have to happen in one physical “space”, the area of which is determined by context and not by the actual size/distance and barriers.

For example, it wouldn't make a lot of sense for a priest to give a blessing to a person in a neighbouring house from his presbytery just 15 metres away, even if the person is receptive (assuming there are no preventions for the priest to go there - it's not a high-security prison, locked-down elderly home etc.) The context is they are in two separate houses, two separate spaces. However, a priest or a bishop can obviously bless all 3000 Mass attendees at once at a congress, even if some of them are physically 150 metres away. They are all in the same space.

If your church is small and you have to stand outside to assist at Mass, you'd obviously fulfill your Sunday obligations and receive the graces of the Mass, even though you're not technically in the church.

I remember a similar topic came up during one of Fr. (now Bp.) Morgan's online catechisms. If I remember correctly, he said that during lockdowns, a good idea to go around the rules might be to have Confessions at a grocery store parking lot, with the confessor sits in his car, and the penitent sits in his/her own and parks next to or near the priest, confessing over the phone to him. This way, they'd be in the same space and aware of each other, and being prevented to speak face-to-face, the phone serves as a communication aid.

The Church allows those with disabilities and to use hearing aids, writing etc in Penance, and priests who don't understand the language of his penitents use a sheet where penitents tick their sins and write down a number.

So I don't think the issue is the phone itself, but whether or not the priest and the penitent are within the same “space” more or less, as much as they are not prevented to be.

https://aquinas.cc/la/en/~ST.III.Q60.A4

St. Thomas distinguishes between proximate and remote "matter" of the Sacrament of Penance. The proximate matter is the speaking of words (this is a sensible bodily action). Those spoken words, as sound vibrations coming from the vocal cords, must be heard directly by the ears of the priest. The remote matter is the mortal sin itself. Both the proximate and remote parts of "the matter" must be there for the Sacrament to be valid.

So a mute person cannot validly be absolved of sin?  What about cases of general absolution where a large group of troops could receive it without having individually confessed their sins, and certainly not to be heard by the ears of the priest.  What if the priest is deaf, say, lost his hearing?  He can't validly absolve anymore?  This criterion fails all these actual examples that are held to be valid by the Church.

Mute person could hand the priest a note.  Penitent could hand a deaf priest a note.  Priests validly absolve a large group of individuals, such as troops, without having heard them (especially those in the back) vocalize any sins.

Having the Penitent verbally confess, or approach for general absolution, or hand in a note proves they are real, present, and a unique person.

These are obscured by using the phone. You may be hearing a recording or synth. You don't know who or what is on the other end; one person, no person, a crowd of pagans, a trained parrot.

You would have to take special measures to have some certainty, just like the air raid scenario, although the measures would be different.

How is a group of soldiers a "unique person"?  One can be morally certain in a telephone call that you're in contact with the correct individual.  If I call my Mom's phone number and she picks up, I'm morally certain that I'm speaking to her and not a crowd of pagans or a trained parrot.

Probably the only principle that remains is that the priest has to see the individual to whom he's directing the absolution.  But then what about penitents who are behind a screen, or even a thick grille as in some cloisters (which the authors admit is valid)?  Or what of a blind priest?

So then it seems to distill that the priest has to know to whom he's giving the absolution and sense the individual through one of his senses.

But then let's take this scenario.  Some mute penitent enters the Confessional behind a thick screen and slides a note to the priest under the screen.  Really the priest can only infer the presence of the penitent due to someone handing a note through the screen.  Priest does not see or hear the penitent, nor does he touch him.  He indirectly senses his presence through the medium of a piece of paper with ink on it.

In terms of hearing an electronic representation of a voice, what if a penitent had a voice synthesizer, kindof like Stephen Hawking and is communicating through that instead of through his own vocal chords?

But in the cases of general absolution, the sins are not communicated to the priest in any fashion.  Nor in the case of a dying unconscious person.

Let's take a blind (but not deaf) priest who enters the hospital room of a dying unconscious person.  He's told by someone that there's a Catholic there dying and unconscious.  He has moral certainty that such is the case and administers absolution to the dying person.  This would certainly be valid.  It seems that it ultimately boils down to the priest's intent to direct absolution to a particular person or (in general absolution) particular persons, whether he sees them or not, senses them or not, or hears them say any sins, or even reads a note about their sins.  He need not have any knowledge of the sins communicated to him (dying person, general absolution).  He need not see or sense the penitent (blind priest in hospital room, mute penitent behind a screen).

And that suggests that absolution over the telephone would be valid.

Which one is it, then, a voice or a physical presence?  It's one or the other that's of essence.  What about when a priest grants general absolution to a large crowd?  There's no individual contact there where the priest is addressing the absolution to any specific individual.  Nor does he hear the voice of any particular individual, or any individual at all.  What about absolution given to a dying person who's unconscious?  There's nothing there but physical proximity ... no interraction whatsoever.

Ok, it's an electronic reproduction, so what?  If a penitent hands the priest a note, the priest is looking at a piece of paper with ink on it, not hearing the voice.

With a general absolution to a crow, there's neither exchange present.

I've yet to see a convincing reason why confession and absolution over a telephone can't be valid.  In fact, even the others admit that it's possibly valid, which means that the principles here are not certain but speculative.

It's another matter that the Church will have to settle, but as far as opinions...

I agree that a phone confession is possibly valid, but that there is still doubt involved.
 
The Sacrament of Penance is essentially a juridical act that sometimes has to take place in circuмstances which are less than ideal, humanly speaking of course.
 
General Absolution is real and valid, but given with the understanding that the penitent will make a good individual confession when circuмstances allow.

Absolution given to the unconscious is always conditional.

In practice when a mute writes his confession and gives it to a Priest, to ensure validity the Priest will verbally repeat what is written and seek some indication from the mute that these are his sins, that he is contrite, and has purpose of amendment.

Let's take another scenario: miners are trapped in a cave in, unreachable, but a phone connection is established. I think a Priest would be bound to hear their confessions and grant them Absolution, but, because at this point the Church has not made a definitive decision, it should be under condition.