Okay, I'm based in the U.K. and I also get unwanted calls (same as described above) from this number... I don't know for certain, but the +20 code is Egypt... That's where I believe this call is coming from. It would also fit with the outside / echo sounds. Whether this helps or not I don't know...
I got a few calls from this number too! Turns out it was from Accenza- the digital agency, which is a venture from one of the founding teams of NASDAQ listed Groupon in Asia. It's the same that helped in Google’s growth in the region.
We were trying to reach you regarding digital marketing services that we offer. This number- +44 20 7193 6574 is one of our registered UK numbers. If you are interested, please visit us at www.accenza.com