Who I have the same questioned this topic

Samsung S7 , my phone will ring once or twice and when I answer the call is dropped.  The person on the other end hears a busy signal if I attempt to answer, if I don't they go to my voicemail.  When I make a call, I hear it ring once or twice and then the call is dropped.  When I am at home I can receive and make calls as long as I am inside so I am assuming the calls are being made via my wi-fi. Help would be great.  Thanks!