For some bizarre reason, when Stella called me just now on my cell phone (O2 Germany network), the number appearing in the display of my mobile phone was not "Zu Hause schnurlos" (an address list entry) but 17325551212—which, I suppose, is Directory Inquiries for area code 732 in New Jersey. Bizarre.
After googling a bit, 17325551212 appears as an example telephone number showing the expected format of a phone number in an email-to-fax gateway and in a presentation on SIP for VOIP.
At a guess, it's the second one that's significant: someone implemented some VOIP service somewhere without configuring it properly and instead transmits the example number from the documentation somewhere rather than the real number.
Why the person creating the presentation/documentation used that particular number as their example, I have no idea. (Perhaps they lived in the 732 area code?)
So I'm guessing that when someone tries to call you using Voice-over-IP (deliberately or because some telephone carrier along the way uses VOIP) and they pass over a broken phone system (one that hasn't been configured properly), you get the example phone number rather than the real one.