You know, in the FISA warrant, it would have listed Manafort and his known FOREIGN contacts, and that is what would have been recorded. If the person Manafort called or received a call from wasn't on the warrant as well, the FBI would have had to disconnect as soon as they verified Manafort was talking to someone other than those on the FISA warrant.
Ever watch the movie "Goodfellas"? It was based on a true story, and they showed how they got over on the wiretapping, because they would have their wives call the other wife, and as soon as they heard the FBI disconnect because they weren't on the wiretap list, their husbands would quickly take the phone, set up a meet, and that is how they got around it.
If Trump wasn't part of the FISA warrant, they would have had to disconnect as soon as they knew it was him and he wasn't part of the warrant.