Pha is the fourteenth of the thirty consonants and the aspirated partner of pa: a p released with a breath. It is not an f, and the h in the transliteration marks aspiration and nothing else, the same convention that produces kha, cha, tha and tsha.
In modern Lhasa speech pa and pha have merged as an aspirated labial with high tone. Pha is a root letter and takes no part in the prefix, suffix, superscript or subscript systems.
With pha the fourth line reaches its second member and the pattern that has run through the velars, palatals and dentals is complete for the labials as well. Four lines, four places of articulation, the same four manners in the same order each time. What follows in the recitation is different in kind: the fifth line gathers the affricates that Tibetan added for itself and the letter wa, and the sixth gathers fricatives and semivowels that fit no grid. The tidy part of the alphabet ends here.
For a learner that is a useful landmark. Sixteen letters in, the system has been shown four times and can be relied on. Everything after pha has to be taken more nearly on its own terms.
Pha takes superscripts and the ya and ra subscripts. Its behaviour under a subjoined ya is the same as that of pa, so a stack of pha with a ya beneath it is heard in modern Lhasa speech as an aspirated palatal rather than as a labial followed by a glide, and the spelling gives no hint of it to a reader who has not learnt the rule.