Tom was on a flight to Pittsburgh Sunday night for a gig, when a fellow passenger snapped a pic of his phone. The screenshot clearly shows there's still a connection between the "Vanderpump Rules" cheatin' duo ... Tom's talking to Raquel.

The passenger who took the shot said it was innocent enough ... Tom had his phone out -- presumably when the plane was on the ground -- so the passenger took the photo, not realizing the person on the other end of the line was Raquel.

The traveler said Tom was trying to speak quietly, but his voice was "highly recognizable." The only thing the passenger heard him say was that he was going to "f***ing Pittsburgh."

There's another side to this ... as we reported, Raquel had been in a mental health facility recently, but typically such facilities restrict the use of phones. The screenshot is evidence she may be out by now.

There have been numerous reports Tom and Raquel are history ... something which Ariana Madix has called BS. Looks like she's right on the mark.

Tom's band is playing at Jergel's Rhythm Grille Monday night.