Under the Patriot Act no (FISA)  warrent is necessary to listen to foreign/ overseas calls. Utter certain "catchwords" and listening becomes investigation. Talk to certain Ruskies and you're being recorded. It's plainly ovious that the FBI/CIA/NSA etc, accidently  or purposefully  caught Trump campaign officials meeting with Ruskies. Whether or not a warrent was needed is moot. They don't need one.