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 Scientific evidences stating Meucci's priority on Bell

1) Anti-sidetone circuit

Meucci began investigating topics related with the telephone ""system"" in 1858, as shown by a drawing of the same year, reproduced below http://www.chezbasilio.it/immagini/corradi_drw.gif. 
This drawing formed the basis for Meucci's caveat of 1871, but, as Meucci's lawyer did not file it with the caveat, it was later notarized by Meucci himself in his affidavit of 9 October 1885.The drawing shows a two-way, simultaneous, telephone conversation, with a complete separation between the two directions of transmission (indicated by arrows). This contrivance is aimed to counteract the so-called ""side-tone"" effect, that is, the hearing of the echo of the speaker's own voice (as well as any background noise) in his own receiver. 
This layout is known today as a ""four-wire circuit"". With it, Meucci anticipated any other inventor, as proved by the following:

No mention of side-tone effects, nor measures to counteract it, are found in Bell's patents, nor in Gray's caveat, both of 1876. In the first Bell's subscriber lines, the four (initially, two) instruments were simply put in series on the same line.
The first anti-sidetone (AST) circuits were introduced by the Bell people in the 1900s, particularly by George A. Campbell, with his nine patents of 1918, many decades after Meucci's solution was shown in this drawing, and over thirty years after the same drawing was notarized.

2) Inductive loading

The drawing reproduced above http://www.chezbasilio.it/immagini/lemmi_drw.gif is taken from a note jotted down by Meucci in his Laboratory notebook on 27 September 1870, and reported in an affidavit by M. Lemmi of 28 September 1885, containing the English translation of said notebook (see also my paper on ""Antenna"").
It can be seen that diagrams ""No. 2"" and ""No. 3"" differ from ""No. 1"" in the insertion midway along the long-distance line of an inductor (with either a horseshoe or bar iron core). In diagram ""No. 4,"" the inductor is split into two parts, inserted at both ends of the line. In his notes, Meucci pointed out that, in this way, he could get rid of the battery, and that, by splitting the inductor, he obtained better results.
This technique is known today as the ""inductive loading"" of long distance telephone lines, allowing to increase the distance and/or the quality of speech. The proof that Meucci came first, can be drawn from the following events, occurred many years after Meucci's discovery (or its notarization):

In June-July, 1887, Oliver Heaviside and Alfred Vaschy predicted, on the basis of mathematical calculations, that telephone signals could be transmitted without distortion by increasing the line's self-induction.
On 19 June 1900, M. I. Pupin obtained the first two patents for said technique. The Bell Telephone Co. acquired both patents and, later on, applied them in its lines

3)Means to counteract the skin effect

Because of the increased bandwidth of the telephone with respect to the telegraph, telephone lines were subjected to increased attenuation, due to the so-called skin effect.
In two notes of 1862 in his Laboratory notebook (notarized in Lemmi's affidavit of 1885), Meucci suggested to treat the surface of the (copper) conductor in various ways (basically with graphite) to increase its surface conductivity.
Moreover, in his caveat of 1871, Meucci stated: “I believe it preferable to have the wire of larger area than that ordinarily employed in the electric telegraph.”
Another note in his notebook, dated 17 August 1870, reads: “To be adopted, for having long distance, bundles of copper wire insulated with cotton or any other kind of wrapper; by this means I have obtained a distance of about one mile.” For this ""bundle"" of copper wires Meucci filed a US patent application ""Wire for Electrical Purposes"" on 2 July 1880.

That Meucci was first in said techniques, is proven by the following facts:

Skin effect in telephone lines was neglected by all inventors till 1880, when it began to be investigated by David E. Hughes and Oliver Heaviside.
Following their studies and the development of ""hard drawn"" copper, the Bell Telephone Co. decided to use copper in place of steel, beginning from the Boston-New York line, which was inaugurated on 27 March 1884.

4) Call signaling

Meucci adopted a telegraphic call signaling, simply effected by a Morse key, attached to each end of the line, momentarily short-circuiting the local transmitter. In this way, strong pulses of current were sent along the line, and the distant receiver would emit intermittent ticks, much louder than the ordinary speech. In his caveat of 1871, Meucci gave a detailed description of his call signaling, consistent w