In the US, if cops get into a phone before a warrant, even if a warrant is issued later, all evidence from the phone is suppressed as illegally obtained.
This is not a hard and fast rule. There is the concept of Inevitable Discovery.
If they find evidence during an illegal entry, they may later seize it lawfully under a search warrant and prosecutors may use it in court if they can supply an independent source, rather than benefit from the earlier violation.
Real world example: In Nix v. Williams, decided in 1984, the Supreme Court considered evidence of a child’s body found after police obtained incriminating statements from Williams. Those statements were later determined to have been illegally obtained and were excluded. However, the body was located in the existing search area and the same type of location (a culvert) they were searching. It was simply in a search quadrant they had not gotten to yet. So the searchers would likely have found the body anyway even without those statements.