Lauren Baker, 19, rang the travel company to check a booking for an upcoming trip to Amsterdam with her boyfriend, Matt Geddes, 22.
Lauren, from Smethwick, rang to clear up a mix-up with her city break booking. But when the operator went to transfer her call, she was mistakenly put through to a sex chat line and she was stunned to hear two people talking dirty.
With the phone on loud speaker, a woman can be heard saying: "Well you're a lucky boy today then, aren't you?"
A man replied: "I think all my dreams have come true."
The sordid conversation carried on for about 45 seconds as Lauren and Matt listened in stunned disbelief.
Lauren said: "We couldn't believe our ears. I realised straight away what it was.
"I was so shocked, I didn't know what was going on - I thought it was maybe some kind of practical joke at first.
"I put it on loud speaker so Matt could hear and we had no idea what was happening.
"I was really confused, both of us were.
"It's a bit weird, but quite funny.
"I rang because I wanted to check exactly where hotel was my emails didn't say anything and the hotel's website said it was in Rotterdam, which is two hours away from Amsterdam.
"I spoke to Thomas Cook and they said that hotel shouldn't have been offered as part of the city break package, but it would cost me extra to change it.
"I didn't want to pay for their mistake so the woman transferred me to the flexible trips department.
"I was put on hold - and when it picked up it was a sex line."
Lauren swiftly hung up, and called Thomas Cook again the following day.
Lauren spoke to the manager, who told her the operator had mistakenly dialled 0844 instead of 0800 and said it was a 'genuine error'.
Matt caught the whole sordid incident on camera, and the couple saw the funny side and shared the video on Facebook.
Lauren, who works as a checkouts operator at Morrisons, said the company apologised for the gaffe, blaming 'human error' but she has been left unimpressed by Thomas Cook's 'unhelpful' attitude.
Although unable to get her money back for the original error, Lauren has since booked another hotel which she said Thomas Cook have refunded for and the couple go to Amsterdam on Friday.