Eru has the ability to speak to anyone, but it would be difficult for incarnates to hear him
Eru communicates via sanwe-latya, direct thought-transmission. This is a technique mainly employed by the valar, but it is also useable by men and elves, just that they'll have a much harder time perceiving nay communication.
Pengolodh says that all minds (sáma, pl. sámar) are equal in status, though they differ in capacity and strength. A mind by its nature perceives another mind directly. But it cannot perceive more than the existence of another mind (as something other than itself, though of the same order) except by the will of both parties. The degree of will, however, need not be the same in both parties. If we call one mind G (for guest or comer) and the other H (for host or receiver), then G must have full intention to inspect H or to inform it. But knowledge may be gained or imparted by G, even when H is not seeking or intending to impart or to learn: the act of G will be effective, if H is simply “open” (láta; látie “openness”). This distinction, he says, is of the greatest importance.
“Openness” is the natural or simple state (indo) of a mind that is not otherwise engaged. In “Arda Unmarred” (that is, in ideal conditions free from evil) openness would be the normal state. Nonetheless any mind may be closed (pahta). This requires an act of conscious will: Unwill (avanir). It may be made against G, against G and some others, or be a total retreat into “privacy” (aquapahtie).
Though in “Arda Unmarred” openness is the normal state, every mind has, from its first making as an individual, the right to close; and it has absolute power to make this effective by will. Nothing can penetrate the barrier of Unwill.
[footnote:]“No mind can, however, be closed against Eru, either against His inspection or against His message. The latter it may not heed, but it cannot say it did not receive it”.
All these things, says Pengolodh, are true of all minds, from the Ainur in the presence of Eru, or the great Valar such as Manwë and Melkor, to the Maiar in Eä, and down to the least of the Mirröanwi[=incarnates, i.e. Elves and Men]. But different states bring in limitations, which are not fully controlled by the will.
The Valar entered into Eä and Time of free will, and they are now in Time, so long as it endures. They can perceive nothing outside Time, save by memory of their existence before it began: they can recall the Song and the Vision. They are, of course, open to Eru, but they cannot of their own will “see” any part of His mind. They can open themselves to Eru in entreaty, and He may then reveal His thought to them.
[footnote:]Pengolodh adds: “Some say that Manwë, by a special grace to the King, could still in a measure perceive Eru; others more probably, that he remained nearest to Eru, and Eru was most ready to hear and answer him”.
The Incarnates have by the nature of sáma the same faculties; but their perception is dimmed by the hröa[=body], for their fëa[=soul] is united to their hröa and its normal procedure is through the hröa, which is in itself part of Eä, without thought. The dimming is indeed double; for thought has to pass one mantle of hröa and penetrate another. For this reason in Incarnates transmission of thought requires strengthening to be effective. Strengthening can be by affinity, by urgency, or by authority.
"Ósanwe-kenta, ‘Enquiry into the Communication of
Thought’", published in Vinyar Tengwar #39
In colloquial elvish thought though, Eru was said to only converse with Manwë, and Elves and Men did not know they had the ability to converse with Eru.
But [the Valar] remained in direct contact with Eru, though they, as far as the legends go, usually 'addressed' Him through Manwe the Elder King. No doubt these legends are somatomorphic (sc. almost as anthropomorphic as are our own legends or imagination), and most Elves, when speaking of Man we appealing to Eru or having converse with Him, imagined him as a figure, even more majestic than one of their own ancient kings, standing in attitude of prayer or supplication to the Valar.
[footnote:]At this time there was no way for the Incarnate direct to Eru, and though the Eldar knew well that the power of the Valar to counsel or assist them was only delegated, it was through them that they sought for enlightenment or aid from Eru.
By nature one of the Valar, or of those of the prime order of created spirits to which they belonged, would be in the presence of Eru only by presenting themselves in thought. The Eldar, and still less the Elves of Middle-earth (and again still less Men, especially those who had no contact with Elves or shunned it), knew little of such things; but they believed that 'direct' resort to Eru was not allowed to them, or at least not expected of them, except in gravest emergency.
"On √PHAN; fana and related matters." - "The knowledge of the Valar, or Elvish ideas and theories concerned with them.", published in Parma Eldlamberon #17