To change the HP number that were assigned from your MBB TAC, there is only 2 ways:

1. Change through ATM Machine using your ATM card
2. Change it through MB2U but you need to get the TAC number first before you able to proceed to the next page.

So if you got a msg from MBB giving you the TAC number but you never requested for it then someone managed to hack your account but is still safe as the hacker won't be able to do anything w/out the TAC(this is what i presume )