In essence the difference between "who" and "whom" comes down to the way they function. Who is typically the subject of a sentence: e.g. Who ate my apple? Who are you? Whereas, whom is used to refer to the object of a verb or preposition: e.g. To whom was this letter addressed? Whom do you believe? A good way to identify whether a sentence requires who or whom is to follow this rule: If you can replace the word with “he”’ or “’she,” use who. If you can replace it with “him” or “her,” use whom. (please bear in mind that there might be some slight structural changes needed to do this). -one thing I will add is the most times in colloquial English whom is not used very often. So from a quick glance these should be correct: Who am I speaking to? With whom am I speaking? To whom am I speaking?

2, 4, 5, and 6 sound too formal. 1 sounds good, but it might be too direct. We might say something like, 'May I ask who I am speaking to?' It sounds more polite like that. As for 3, I don't think it's suitable to use with unless 2 people are having a conversation and then the person suddenly wants to know the name of the person. I think the preposition 'to' sounds better and is more suitable for a phone call.