They may not have needed a warrant to monitor Trump. Based on something I read in one of Sharyl Attkisson's articles, such warrants as the one on Carter Page may have authorized them to monitor communications "two steps removed." That is, they could monitor the communications of anyone talking to Page, and then of anyone talking to those sources. Such a reach, if I understand it correctly, could involve monitoring hundreds of sources, including Trump and everyone in his campaign.
Obama administration officials could deny tapping Trump's phones, while at the same time they could be intercepting everything he was saying.
They get a warrant for someone near you and that allows them to listen in on all his contacts - indirect warrant-less but “legal” bugging.