Hi this is Dr Shabd
The only and easiest way is
1-Go to Setting - Switch off the Face Time
2-Go to Settig - Switch off the iMessage
3-Go to Setting - General Settings- Reset Network Settings
4 - Go to Setting Switch on FaceTime - Sign in with your existing Apple / iCloud ID
You can see the Mobile number is changed now.
For more queries do write me or Whatsapp me.