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 figure out why this call was even happening.

Sombr did his best to play it off -- insisting it was all just a harmless joke meant for Selena -- but Benny clearly wasn’t having it. The vibe was ice cold ... and Sombr eventually had to hang up.

Luckily for Sombr, even if Selena’s hubby isn’t exactly in his fan club, her BFF Taylor Swift definitely is -- he once said he “made it” the moment Tay admitted she had his tracks on repeat!