My god! I've been hearing this really specific tune for ages in the middle of the night, I always assumed it came from the neighbours house (still think so) and today I heared the same sound, twice! Once while I was in the bathroom cleaning up (from the neighbours house) and once while watching the big bang theory, when Priya called Leonard over skype (from the tv).
What made my day about this: I finally identified the sound! After I've been hearing it for two years and never knowing what it was. It's a Skype ringtone! So glad I can finally stop thinking about this.
Now the question is: who is calling my neighbour at all hours of the night (I usually hear the sound between 11pm and 3am, mostly between 12.30 and 2.30)