Question about legalities and Cannabis?

In Wi.....Situation. Party A is upset about a incident that occured between them and party B the previous month. Party A has been using implied threats about party B's drug use. Party B is concerned that Party A will call police about this use, simply to "get back" or in other words F***ing snitch. Is this legal for Party A to do, and can the Police legally search a residence because of one phone call? Thanks for the help, i hate government