Sombr thought he was dialing up Selena Gomez for a cheeky prank ...  but instead, it was her husband Benny Blanco who answered -- and the awkwardness was off the charts. It all went down during Sombr’s "Call Your Ex Onstage" bit -- but a playful jab at Selena turned weird when Benny picked up … and was straight-up baffled about why he was ringing him. You’ve gotta see the whole thing play out ... we can’t hear Benny’s side, but from what it looks like, BB was clearly grilling Sombr, trying to