I would like to know, when making a telephone call. Do we say "May I speak with so & so or do we say "May I speak to so & so?
Your response would be appreciated
Either "to" or "with". Both are fine. As for meaning, "to" expresses that you, the speaker, will be giving information. The information will be going in the direction of the listener, to(wards) them. "with" expresses that you, the speaker, will be sharing information with the other person. You will speak together. The difference between "speak to" and "speak with" is very slight, but nonetheless some speakers known intuitively that if you want to soften a request use "with" because it expresses sharing of ideas--a conversation.