The world championship leader raised some smiles in second practice at the Spanish Grand Prix when his response to a radio message about his run plan regarded a ringtone that could be heard from the garage.
Verstappen was able to recognise it as the one used by Red Bull's motorsport advisor Marko.
"Someone's phone is going," said Verstappen. "Is that Helmut's?"
The team's subsequent confirmation raised some smiles, and the moment confirmed in Horner's head the high level that the Dutchman is operating right now if he is able to pick out small details like that while driving an F1 car on the limit.
Reflecting on the progress that Verstappen has made since his maiden F1 win at Barcelona in 2016 to be currently dominant in F1, Horner said: "I think in 2016 he was very raw. He was incredibly fast, hugely, and naturally talented.
"He still has that natural speed and tremendous ability. But I think what he has now is the experience, roundedness and the [mental] capacity.
"When he's recognizing Helmut Marko's phone ringing from within the car, he's just got this added bandwidth that has put him on another level, and I think the exciting thing about him is he's still getting better."
Not all of Verstappen's radio chatter with engineer Gianpiero Lambiase is so light-hearted though, and at the end of the Spanish GP it appeared that the Dutchman ignored a request not to go for the fastest lap.
Having already been given a black-and-white flag for track limits breaches, Lambiase did not think it was worth the risk of pushing hard and potentially getting a penalty for one extra point. But Verstappen ignored him and duly delivered the fastest lap.
Horner did not see any particular problem with what happened, as he likened the relationship between driver and engineer to that of an old married couple.
"His engineer was just informing him of one more strike and it's a penalty, so not to take any risks in the fastest lap," said Horner.
"The relationship they have is one like an old married couple, almost debating about which channel of television they should be watching.
"But Max was totally in control. He was aware of the risk and was still able to do the fastest lap quite comfortably with the tyres that he had on the car."
