I always put n/a on any form where it asks for my moble number. I also de-registered from the directory, took about 30 seconds all told.
30 seconds 
Took me about 10 minutes 
I also managed eventually to get delisted from T-Mobile's promotional text service (see thread in the Computer Room).
That took a bloody age too after many auto re-directions pressing different numbers.
Hi Mr Pastis, may I call you by your first name?
No! Just stop sending those promotional texts will you!
Ok Mr Pastis, we'll just have to go through a security check first, if you don't mind
Do I have an option there too? 
I'm sorry Mr Pastis, but we're obliged to carry out a security check
And on, and on, and on it went ... 
And my personal favourite:
Them: "Will you confirm your telephone number?"
Me: "Of course"
Long silence ..............................
Them: "Hello????"
Me: "Hello"
Them: "Will you confirm your telephone number?"
Me: "Of course"
Them: "Well?"
Me: "You tell me what number you have for me and I'll confirm whether you have the right number"
Them: "That's not how it works, you have to confirm your number to me"
Me "I understand that but I cannot confirm it until you tell me what you think it is, then I can confirm whether it is right or I can tell you if it is not"
Them: "You don't understand sir ~ I am trying to confirm that we have the right number for you ~ it's for your security"
Me "It is even more secure if I don't tell you my telephone number don't you think? BUT I will confirm whether you are right or wrong if you tell me what number you have for me"
Long pause
Them: "Hello"
Me: "Hello"
Them: "Are you going to confirm your telephone number?"
Me: "Yes once you tell me what number you have, only then can I confirm it ....... unless this is a convoluted way of you trying to ask me to tell you my telephone number which you don't have and that I'm not going to do because that is not confirmation of a number you already have that is supplying you information you do not have. Not getting very far are we?"
Them: "But it is for your security"
Me: "You let me worry about my security sunshine and you worry about your misuse of English ~ Goodbye ....... click Brrrrrrrrrrrrrrrrrrr."