We've seen stupid questions on here before, this is not one of them.
The main reason that Cunard request a mobile number is that nowadays most people can be contacted throughout the working day on them whereas the home 'phone is often unmanned at that time. So should for any reason they need to contact you prior to embarkation they have a much better chance of doing so via your mobile ..... Neil
Yes most cruise ships are or have installed the equipment but it is not cheap, but OK for texting and speech in emergency, unless you don't bother about cost.
You will need a tri band phone at the very least to connect to the on-board system but i warn you it is very expensive .