Orlando, 42, was appearing on BBC Radio 1 when a call came in from a woman named 'Gemma' who was a self-confessed super fan of his musical wife-to-be.
Socially awkward Gemma - who had a thick London accent - then recited Katy's track, I Kissed A Girl, without much conviction.
Unimpressed Troy star Orlando tried his best to be kind about the performance, only to stumble over his words.
At which point Gemma revealed herself to, in fact, be Katy.
'If we're going to be going down this long road [marriage] you'd think you'd be able to recognise my voice,' Katy taunted the actor.
Over on Radio 2 the next day, Orlando spoke candidly about the moment he popped the question to Katy while promoting his new series with with Cara Delevingne.
He revealed how his proposal didn't go as smoothly as planned, as the box containing the engagement ring got stuck in his jacket pocket.
Discussing the mishap with Zoe Ball, he reminisced: 'It wasn’t too bad, I just couldn’t get the ring out of my jacket… I had a premonition because I said to the guy who I bought the ring from.
'He's a friend and I said just make sure the ring box isn’t too big. He said it will be fine and I just knew it, it was too big clearly because I couldn’t get it out of my pocket.'
The lovebirds have been dating on and off since January 2016 after they were seen flirting with one another at the Golden Globe Awards in Beverly Hills.
Meanwhile, Orlando's Carnival Row co-star Cara denied having wed girlfriend Ashley Benson, after The Sun reported earlier this month that the celebrity couple wed at the Little Las Vegas Chapel.
When asked if she should be congratulated, Cara systematically denied it by saying 'no' and adding that 'it is indeed' nonsense.
Cara and Ashley were first linked together in May of last year after starring in the Elizabeth Moss drama Her Smell.
Despite evidence of a relationship, they stayed mum, only occasionally featuring each other in social media posts and never referencing their relationship.
Katy and Orlando first revealed their engagement in February less than a year after rekindling their romance where they took time apart to give each other 'respectful, loving space.'
While the couple did not fully confirm their engagement, Katy's mother posted snaps to Facebook from what she described as their 'engagement party.'
Earlier this year, another source told People that the famous couple were 'staying secretive' about their big day'.
Sparks flying: Katy and Orlando have been dating on and off since January 2016 after they were seen flirting with one another at the Golden Globe Awards in Beverly Hills
Romantic: While the couple did not fully confirm their engagement at the time, Katy's mother posted snaps to Facebook from what she described as their 'engagement party.'
During an appearance on Good Morning America earlier this year, Katy said, 'Someone decided to put up with me for the rest of my life!'
But she did not share a date: 'Well, I guess I should get started on that! I haven't figured that part out. We're definitely, like, enjoying the moment.'
Roar singer Katy was previously married to actor Russell Brand, after a lavish ceremony in India in 2010. The couple divorced two years later.
Orlando was wed to model Miranda Kerr from 2010 until 2013. Together they share son Flynn, eight.
