Does the emission of pre-seminal fluid—and subsequent white drops that are possibly semen—due to a lust-inducing phone call while fasting necessitate expiation (kaffarah) or simply making up the missed fast (qadaa'), and is this considered a major sin, and will repentance be accepted for this sin?

The conversation you had with the foreign woman is a great sin, and its sin is more severe in Ramadan due to the sanctity of the time. You must repent from it, though it is not among the major sins. The fluid you saw is most likely madhi (pre-seminal fluid). As for the two thick drops, they are most likely wadi (thick white fluid that may be discharged after urination). Even if it is assumed that they were mani (seminal fluid), its discharge does not affect the validity of the fast according to the majority of scholars.