Your number will only be removed if it proves to be inactive over a period of at least 6 months, with 3 attempts made. No hard-and-fast rule. Plus once a company has bought your information from someone who claims your info matches their target audience, they are free to attempt to sell it on to other companies. Not answering the witheld is generally the best thing to do (you just gotta hope your info isn't passed around too much) for sure, but there's no guarantee it'll be the end of it.
... Yeah I work in a call centre. B2B! Previous customers only! ... So our number comes through as witheld. I'm amazed how many people answer the phone then whisper "i'm in a meeting!" -- just don't answer! Then again... my mum has her number 'witheld', so I can't tell if it's her calling or 3 mobile trying to convince me to get my friends and family to sign up too.
I hate my job.