Hmm, I have the same problem too. I tested it a couples of time, if your contact number is like +17271234567 then I will get unknown. My other contact with 1(727)123-4567 then i will not get the unknown and get the correct caller ID... i think it is definately bug,

Try to change the contact add "(" or ")" or "-" sign, it dont allow for those contact with only +7271234567.

Hope Nokia i will fix it.