Who decided to spell “phony” or “phoney” that way and why? Usually, a “ph” can be traced back to a Greek φ (phi), but not so here. Wiktionary says it may come from “fawney”, with no Greek in sight. Was it to seem more learned? That would make the word itself a bit pretentious or foney, sorry, phoney.
The following site suggests the origin of the term phoney citing lexicographer Eric Patridge and the OED. As for the spelling, they suggest that the use of ph- vs f- was probably influenced by terms of Greek origin which, like telephone, were first used at that time:
The origin of phones was via American phoney man, a peddler of imitation jewellery.
Partridge notes that phoney man is:
from its original, the English fawney man, itself an adaption of the British fawney cove, one who practises ‘the fawney rig’ or ring-dropping trick, involving a gilt ring passed off as gold and first described by George Parker in A View of Society, 1781.
While many etymologists suspect this origin of phoney is genuine, we still not absolutely certain of its truth. If it is true, the spelling of phoney, using ph– for f-, must be influenced by spelling of the Greek-based phone, I imagine. My speculation fits the historical timeline: phone, short for telephone, is recorded by 1880, while phone, as a speech sound in linguistic circles, is documented a little earlier.
And phony‘s passage from Irish to British and American English also generally matches with the Irish diaspora – though I, as a person of Irish descent who is soon moving to Dublin, must take umbrage at the aspersions phony’s origins casts on the Irish.