Hi there, I had the same issue. I arrived in June 2016 and finally received my GC around Oct/Nov. I think there are delays right now in USCIS because their IT systems are giving them trouble. I saw some news a while back that several green cards were mailed to wrong addresses due to system errors. Who knows if yours was one of them. But don't worry too much. I am just speculating and you don't need to worry about their errors because it's not your fault. Still, I think it helps to keep yourself informed.
Sadly, their so-called receipt number of immigrant visa fee is a myth to most of us. I printed out and kept a copy of my fee payment confirmation page but the only thing it has are Pay.gov tracking ID and agency tracking ID. There was no receipt number given after I paid the fees. Since yours is already over 120 days, you might want to tell them the dates you paid your fee and request them to give you receipt number, or ask to speak to a higher level officer. The people who first answered your call are from helpdesk who are helpful but cannot help you beyond what you can find out from USCIS website yourself.
Another way to try to get receipt number is to go the same site you paid your immigrant visa fee and find this on the left hand side:
Already paid? No fee required?
Verify Your Identity
After you verify your identity, it might give you the receipt number.
In my case, they refused to do anything until their stated "120 days" is up, i.e 120 days after I entered the U.S. But the good news is that you don't need to wait for your GC to apply social security and driver license, which are the essentials to get yourself settled imo. Before you get your GC, you can use admission stamp on your passport (if you entered with an immigrant visa like me) as an equivalent to apply SS and driver license, which I did successfully. After you get those, your life here will start going and so, you can sit back, relax and wait for the slow grind of bureaucracy to work its magic. The problem though is many government employees are not aware that one can apply SS and driver license with passport stamp (or pretend to not know that. I am sure most are simply ignorant without any ill intent but there are definitely some bigots out there). So you might want to research and arm yourself with documentation from US government official websites that your passport with immigrant visa admission stamp is a valid proof of residency, in lieu of the actual GC. If the officer denies your application, try to politely point that out and maybe hand them the printouts from websites. Still, some might simply don't know how to "enter it to the system". Then, visiting another service center might help (in my case, I tried visiting another SS office in the region and the officer proceeded without a hitch).
An example of website that says admission stamp on your immigrant visa is an acceptable proof of residency:
(sorry, forum won't allow me to post links. So google this text "ssa - poms rm 10211.025 - evidence of lawful permanent")
B. Acceptable evidence of LPR status
"Machine Readable Immigrant Visa (MRIV) with Temporary I-551 notation"
Hope it helps and happy new year 2017!