It's been my experience that if requested you may select from perhaps 5-10 random numbers when signing up for new service. Some representatives are more cooperative than others. You may even have the option of requesting a vanity number. Phone numbers usually may be changed at any time. Each carrier has their own policy regarding charges for these types of requests.
I believe cell phone number portability becomes effective November 24, 2003. This allows you to transfer your existing cell phone number to your new carrier! It's about time, IMHO.
Good luck.
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The wealthiest person is a poor man at times, compared to the man with a satisfied mind.