As stated on quora.com, to get a new Straight Talk Wireless number, you should call 611 on your phone and tell customer service rep you are getting too many unwanted callers/telemarketers. They should give you a new number at no charge at all. The Quora answer also states that you may call 1-87*-430-2355 one of the options on the menu is to change your mobile number. This will assign you a new number from your activation zip code. Then you should turn the phone off for about 5 minutes and the number will be charged. This process is irreversible and your old number will be permanently lost.
To get more information about the steps on how to change the number, please call the Straight Talk Wireless customer care team by phone at (877) 430-2355 (toll-free number).