> Obama would've had to have sign any FISA warrant?

No, the straw man argument is that we can tell whether Trump was wiretapped by the presence or absence of a FISA warrant.

The president does not sign FISA warrants, the entire purpose of FISA is to streamline (almost to automate) the issuance of warrants.

https://en.wikipedia.org/wiki/United_States_Foreign_Intellig...

My understanding is that "listening" to communication between Trump and a foreign national would not require a FISA warrant. I say "listening" because, I believe, all communications with foreign nationals are recorded but only some are actually listened to by actual people.