What is the meaning of this sentence 高橋ですが、今よろしいでしょうか, is が being used to indicate that the speaker has more to say. Also what is the meaning of でしょう in this sentence?
The が here is kind of a softener/bridge/segue to the actual relevant clause which follows the first. Oftentimes this is conflated with が meaning 'but', however it is not always appropriate to translate it thus. You will see this use of が quite often in sentences such as すみませんが、トイレはありますか？ 'Excuse me, is there a restroom (here)?'. This site describes it as an 'introduction particle' and provides a few more examples.
でしょう expresses uncertainty in this case, as simply asking 今はよろしいですか might sound overly direct. Think of this as similar to when English speakers use 'Might now be a convenient time?' rather than 'Is now a convenient time?'. See the top SE answer here for a bit more detail.