Is what happened between me and my wife before marriage, in terms of sexual practices over the phone and a promise of marriage, considered zina (fornication/adultery) and unlawful?

A woman becomes lawful through a valid, Shariah-compliant marriage that fulfills all conditions. What you mentioned about a marriage between you that is "only" that is not a valid Shariah marriage. Your relationship before it was forbidden, and you both must repent. This relationship is not zina (fornication/adultery) in the specific sense that necessitates a legal punishment (حد), but it is zina in the general sense mentioned in the Prophet's (peace be upon him) hadith: "The son of Adam has his share of zina written for him..."