I was on this merry-go-round over 1 year ago, residing in N Ireland I phoned the number I was given to start process, after a series of questions I was told I did not need an NIS, well then I could not file my Self Assessment, so a week latter I phoned number again, went through series of questions and told the same thing,
then I sent an email, explaining my situation (Spouse Visa), had to pay taxes, etc. and got a letter (wish I could post letter but I am in states now and ltr is in NIreland) bottom line, no NIS. After some time passed I called NIS number again, prepared to record conversation, this time I talked to someone who knew what he was doing, he gave me number to NIS in Magherafelt NI, I made an appointment and got my number, this story probably did not help you but morale of story is you need to talk to someone who knows what they are talking about.