Please contact your cellular service provider OR visit their customer website in regards to changing your phone number.
For post-paid (contract) carrier (like AT&T, Verizon, Sprint, ect.), they can change your number in a few minutes. HOWEVER, there will be a charge added to your bill for the number change & your next bill will look a bit weird, but will balance itself out.
* For those paid in advance (paying for the month you will be using services), you'll see a partial credit on the old number (for time unused during the previous bill cycle) that gets canceled out by a partial charge on the new number (for the days it was active the previous bill cycle).
* For those paid in arrears (paying for the month you just used services for), you'll see a partial charge on both the old number & the new number that will tally up to your normal monthly fee (with a possible exception of regulatory charges).
Please be aware that the carriers may waive the number change fee in certain cases, but it'll be at the customer service rep's (or their supervisor's) discretion... so if you're a good customer that doesn't get a lot of adjustments (bill corrections excluded), they may waive it for you. Therefore, be nice to the rep that you're talking to, as they're not required to waive that fee (but may do so as a one-time courtesy). Being rude or mean to them will make them resistant in waiving it (people generally don't go out of their way for jerks). Escalating to a supervisor may not help as well, since supervisors & managers are not required to be nice to customers.
Calling will give you limited options on what your new number will be (as service reps have to maintain a certain average handle time for calls & cannot spend a lot of time going over all your options).
Doing the number change online will give you the most control over your new number (as you can select among the list of numbers provided), but you'll have to pay the fee.
For those on a prepaid service (Straight Talk, Total Wireless, Tracfone, Boost Mobile, Verizon Prepaid, AT&T Prepaid, ect.), the process is generally the same... HOWEVER, you may have to pay the fee upfront OR lose some service time in lieu of payment. Again, check with your service provider for all of the details.
It is IMPORTANT to note that you are NOT REQUIRED TO CHANGE SIM CARDS! Your carrier can assign a new wireless number to your SIM card within their systems for your account... so you don't have to get a new SIM card at all. The only time a new SIM card would be needed is if you're changing phones & your current SIM card is too old (only support 3G networks & you're going to a 4G LTE phone) OR is the wrong size (have a micro SIM & your phone needs a Nane SIM).
Please be aware that once you change your wireless number, you will need to reboot your phone (turn it off for about 15 seconds & then turn it back on) to properly reflect your new number & to restore service. If you are changing your wireless number with a customer service rep, it's HIGHLY RECOMMENDED that you call from a different number to avoid getting disconnected as service for the old number will be canceled immediately after the new number starts service. If you cannot contact them through an alternate number, they will advise you to reboot your phone once the call drops BEFORE they finalize the change.
I must note that your old number will be held for 59 days, in accordance your carrier's cancellation policy (as their system treats a number change as activating a new line & canceling the old line at the same time... but without any of the standard activation & cancellation charges), so you could revert to your previous number if you change your mind. After this time, your number gets released back into the pool of available numbers for others to use (which you could potentially reclaim it in the future). IF YOUR PORTED YOUR NUMBER FROM A DIFFERENT CARRIER, your old number goes back to it's original carrier & is permanently lost with your current carrier.
Hope this helps!