You guys can actually both be right here. Without a name or other identifying info it makes identifying the guy very hard, but not impossible. On the other hand it's been well established that the FBI has been provided marching orders that now err on the side of caution due to "islamaphobia". How serious they took it determines how many resources they were willing to put into it and how far they were willing to go.
They could have dumped the numbers from the nearest cell tower (or all nearest) in the 1 hour window when the cell call was made and then run those numbers against a database of known investigated phone numbers. I'm sure they had this dudes cell number in their database already (or should have). Now, if this guy had poor opsec, which is usually the case, then bingo they'd have gotten a hit and Id'd the guy.
But doing this is a huge pain in the ass and usually reserved for things that are being taken very seriously and normal random tips probably don't rise to that level. After the Boston bombing or 9/11 or San Bernardino? Yes.
Here, who knows? Hindsight is 20/20.