That が is not the subjective case particle, but the conjunction. It suggests the last half of the sentence is omitted.
The nuance depends on the context.
６時にお会いする約束ですが、取り次いで頂けますか。
I have an appointment with him at six, could you call him?
６時にお会いする約束ですが、聞いてらっしゃいません か。
I have an appointment with him at six, didn't you know that?
Anyway, this が somewhat urge the listener to answer.