Most Mesropian letters are so different from the characters of other alphabets, that it is often impossible to establish their prototypes. In this respect, the form of letter Փ is rather unexpected, because its resemblance to the Greek Փ is so obvious that almost all researchers are of the opinion that the Greek Փ is the prototype of Armenian Փ. However, despite the visual similarity, they have a different phonetic value: Armenian Փ denotes the sound [p], uttered with aspiration – [ph], while the Greek letter communicates sound [f]. This is because there was no phoneme [f] in the Armenian language of the V century, and the Armenians did not discern these two sounds (in some dialects there is no [f] even at present; please see the details in the article about letter Ֆ).
Perhaps, in the ancient times people had a special attitude towards letter Ф? Some light on this question may be shed by a Coptic mystical and philosophical treatise “On the Mysteries of the Greek Letters” (VI century). The author of the treatise – Sabbus the Sanctified interprets the Greek Ф as a letter consisting of two parts – a circle, which signifies the world, and a vertical line that symbolizes the Christ. The part of the vertical line, which rises above the circle, points to the Ascension of Christ, and the one that is lower – to his descent into the hell. According to the Christian dogma, after the crucifixion Jesus Christ descended into hell, liberated the imprisoned souls and led out from hell all Old Testament saints, and also Adam and Eve. Drawing such a high vertical in letter Ф, did the ancient scribes seek to emphasize the great distance between the heaven and the hell? Unfortunately, this question is still awaiting its response.
See more at Aram Khachaturians "Armenian Alphabet. History and Symbolism" book