I see three possibilities:
1. There was a FISA warrant and if we’re lucky it will become public (even though they are normally sealed)
2. There was a warrant that was signed by the AG (the crooked Loretta Lynch) authorizing the survillance without the FISA court’s approval. I’m no lawyer but the pertinent law is 50 USC 1805 that makes it seem that this is possible. Even wikipedia says there are exceptions to FISA authorization “with approval of the AG”. But even in this case there would be a paper trail.
3. There was no warrant and the surveillance was illegal as you suggest. If so, we may never know the truth because there would be no paper trail.
Since it’s a virtual certainty that the intercept was done on a digital line, there has to be an order authorizing the phone company, cable company, ISP, etc to enable data from the devices under surveillance to be sent to the monitoring location. They do this all the time (lawful intercepts for law enforcement purposes) and there’s a prodedure. You can’t just slip Louie the tech at the phone company a few bucks and have him turn on a wiretap. The procecure has checks and balances and is carefully controlled to preserve the integrity of the evidence that is collected. That means there will be a paper trail, although it may take a special prosecutor to uncover it. Paging Rudy....
former Obozo secret service agent spilling his guts this week.... http://www.bizpacreview.com/2017/03/06/fmr-obama-secret-service-agent-bongino-shares-info-obama-wiretaps-blow-wide-open-455846
4) there was a FISA warrant but its limits and/or targets were exceeded.
5) There may have been a warrant that did not reference Trump, Trump Tower, or possibly even anyone in Trump’s circle.... i.e., a warrant for surveillance on one or more Russian banks or persons. It could well be that someone in the intel agencies exceeded the lawful limits, it’s not like that has never happened in history!
You got it. A big paper trail. And if that exists you can bet the applications were falsified and or wrought with omissions.