The major questions the Faults center would ask you are:
Have you tried removing all extensions and equipment from your line and left a corded phone in the main socket? If that didnt work, have you tried another corded phone in the main socket?
If yes, check :
When you dial out, does the other person see your number on their display? If not, you may have a crossed line and you need to speak to Faults to get a tech out.
If they get your number on the display, check:
Have you checked the volume controls on your phone to check they are set to high (if you have the option to)?
If yes, finally try:
And finally, have you changed your number over from BT recently? Has this occured only after the number port has gone through? If the problem has occured since the Number port went through then it is a problem with portability -again you'd need to speak to the Faults centre.
Hope these suggestions helped, they are standard questions for every faults agent, so it may save you some time on the phone if you do need to call NTL.
If you still have a problem, call faults on 0845 454 0000 Option 3
The best of luck