Warrants are pre-trial activities to collect evidence, defendants (or, more likely, their attorneys) are still able to challenge the admissibility of evidence should a case go to trial. If it turns out a search warrant was requested in bad faith, or the trial judge (which won't necessarily be the same one that signed the warrant, and then there's appeals courts) finds the warrant was defective (overly broad, lack of probable cause, etc.) and should not have been issued in the first place then any evidence stemming from it could be thrown out.

"Fruit of the poisonous tree" simply means the entire chain, the initial evidence that was improperly acquired and anything that was discovered based upon it, gets thrown out. If a warrant was issued to dump the full contents of your phone, and they used location metadata from your photo library to start determining other locations to search and got warrants for those, then that entire chain of evidence gets thrown out if the court finds the initial warrant for your phone was invalid.

> If they have a warrant to the phone, what is poisoned fruit?

Litigation over the process, claims, and ultimately validity of the warrant happens after it is executed, where the product is used in a criminal case (one of the commonly argued problems with the FISA warrant process is that, because the products are not used in criminal cases, this never occurs, and because it is known that it will never occur, the constraints that the possibility of challenge places on both the conduct of executive agents seeking warrants and judges granting them also is missing.)

If they have a warrant that adheres to the particularity requirement of the Fourth Amendment, then anything they might find that it outside the scope of the warrant would be illegally seized. For instance, if the search warrant were to say "search the contacts on the phone" and they go looking for pictures.