I read a lot about the problems with UNKNOWN being displayed on iphone 6 and 6s even for numbers in the address book. Almost everything pointed to HD voice or Voice Over LTE (volte). This was the 4th phone on my account and caller ID was a must. So I called technical support and he solved it in less than 5 minutes. Turns out this phone did not have the caller ID feature attached. Now it works perfectly!
Thank you technical support!