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This may shed some light on the matter. It was a question considered during WWII, if a parish priest could given a general absolution to the Catholics of a town whilst in their homes during an air raid.

Questions and Answers: The Sacraments (Canon E. J. Mahoney D.D.):

Is a general absolution valid if given by a priest in the presbytery to all the Catholics of a town in their homes, or wherever they may be, at the beginning of an air raid?

(i) A general absolution to be valid must conform to the requirements of this sacrament iure divino, which are all contained within the teaching of the Church in the Council of Trent, Sess. XIV, can. 9, that priestly absolution is a judicial act. The essentials of this act, as well as the conditions for the valid reception of any sacrament, are certainly observed when a general absolution is given to a regiment or to a church full of people: (a) the penitents have the requisite intention and manifest it externally by reciting the act of contrition; (b) the judicial sentence is pronounced by the priest in the words of absolution, after reminding the people, as ordered by the Holy See, that the absolution is of no avail unless they are rightly disposed, and that an integral confession of their sins must be made on a future occasion; (c) the recipients are present at this judgement.

(ii) In the circuмstances of the above question, the faithful could be previously instructed to form an intention and make an act of contrition immediately the sirens sound. At the same moment the priest could pronounce the words of absolution, having previously instructed the people on the conditions attached to its reception, thus securing some degree of simultaneity between the matter and form of the sacrament.

If, in addition, it could be said that the recipients are, in some probable sense of the word, present at the priestly absolution, it would follow that the act is probably valid and therefore permissible at least conditionally. It is under this aspect of the matter that many disputed questions have been, and still are, discussed: absolution by letter is invalid; by telephone extremely doubtful; pronounced over one who has precipitately left the confessional, it will depend on the distance. Whether examined on theological principles or subjected to a tiresome casuistical method, a correct solution of all these questions turns on establishing the presence or absence of the penitent at the moment of absolution.

If it were merely a matter of ecclesiastical law, as in the absolution of censures and other penalties, the judgement could validly be given in a variety of ways: by letter, messenger, telephone, telegraph or wireless. Nor is there anything in the nature of a judicial sentence which necessarily requires the presence of the penitent at the moment of absolution. This is require in the sacramental absolution of sin because, from the teaching of the Church, Christ has willed that the judgement shall be given in words and words alone: "Docet... sacramenti poenitentiae formam, in qua praecipue ipsius vis sita est, in illis ministri verbis positam esse: Ego te absolvo, etc." "Forma huius sacramenti sunt verba absolutionis."

This verbal form, in which the penitent is mentioned by the personal pronoun, necessarily implies that he is present when it is uttered, and the theologians, therefore, restrict their discussions to determining the outside limit of distance within which "presence" is verified. They write, indeed, of "moral" presence, but we agree with Chrétien, that "physical" presence would better express their meaning; a person would be morally present in his deputy or proxy, which is clearly insufficient. It may be held with St. Alphonsus, that penitents are present if they can be seen priest, but it accords better with the vocal form if we require the voice of the priest to be heard. Thus a well-established probabilist like Noldin advises that, in giving general absolution to an army, the men should be divided and absolved in separate groups if some are too far distant to hear the priest's voice.

We cannot find any writer who expressly deals with the above question, and we are not aware of any arguments proving that these scattered penitents may be considered present. It could be maintained, we suppose, that they are present as a body of parishioners. But in country districts the parishioners might be spread over a vast territory, and if these can be absolved, why not the whole diocese or the whole nation? Since we can find nothing to justify the view that penitents in these circuмstances may validly be absolved, nor even a probability in its favour, it is our opinion that absolution may not lawfully be given even conditionally.

Cf. writers De Poenitentia: de modo proferendi absolutionem, e.g. Cappello, II, §92; Marc- Gestermann, II, §1663.

We cannot find any writer who expressly deals with the above question, and we are not aware of any arguments proving that these scattered penitents may be considered present. It could be maintained, we suppose, that they are present as a body of parishioners. But in country districts the parishioners might be spread over a vast territory, and if these can be absolved, why not the whole diocese or the whole nation? Since we can find nothing to justify the view that penitents in these circuмstances may validly be absolved, nor even a probability in its favour, it is our opinion that absolution may not lawfully be given even conditionally.

It helps some but doesn't really clear it up.  This matter seems like it remains fairly disputed.  While it's stated that absolution by telephone is "highly doubtful", doubtful does not mean certainly invalid, making it unclear what the exact principles involved are.  If the principles were completely clear, it would just be straight invalid.

It does try to make some argument from the fact that a juridical act require the presence of the individual.  But does it?  People can be tried in absentia.  People are excommunicated without being present, etc.  I find this unconvincing.

As he admits, the writers speak of a "moral presence" but then he says that this moral presence is BEST reflected by a physical presence, and here he write that it "could e maintained, we supposed that they are present [aka morally] as a body of parishioners".

So the notion of presence seems to have some dispute about it.  I'm not sure how the presence of communicating with a priest over the phone wouldn't be considered at least a moral presence.  If I pick up the phone and I'm talking to my Mom, even though we're physically separated, there's certainly a moral presence there.