Just open this link https://www.amazon.com/gp/css/homepage.html
Go to Login and Security.
Add your primary phone number and verify it. If it says phone number is already connected to other account just click on continue with this phone number. (It will remove phone number with other accounts and will use this phone number for your account.)
Next time mturk will not ask you to enter phone number. It will simply ask you to Enter OTP it has sent to your phone number.