OEM = Original Equipment Manufacturer
Carrier = cellphone carrier (ie, AT&T, Verizon, TMobile, etc)
If you've got a Pay As You Go phone, it is most likely privately branded to a carrier. The carrier most likely modified the version of the windows phone OS, so the carrier that privately branded the phone is the entity to whom you should direct your complaints.
Thanks for clarifying. However I bought my phone SIM free (from Amazon as I said) so no carrier involved I guess. Presumably therefore the features that Entegy has on his Lumia 520 would have been removed by either Nokia or Windows so that Amazon could sell
it on cheaper?