Is it permissible to speak on the phone with a suitor who is not yet bound by a legally or civilly contracted marriage, and would that be considered a breach of a covenant I made with Allah not to speak with non-mahram men?

A woman's covenant with Allah to refrain from addressing men who are not her mahram is considered a vow (nadhr) and an oath (yameen), and it is a commitment before Allah. A vow, in Islamic law, is when a legally responsible individual obligates themselves to something they possess. A vow is stricter in ruling than an oath. If a Muslim vows to perform an act of obedience, they are obligated to fulfill it, based on the Prophet's (peace be upon him) saying: "Whoever vows to obey Allah, let him obey Him." A woman's refraining from addressing men who are not her mahram is an act of obedience to Allah. The suitor is not a mahram to his fiancée, so she must fulfill her vow and not address him. The way out of this is to expedite the marriage contract.