If it is a particular person who keeps getting transferred to the mailbox, you have inadvertently placed this person on the blocking list.
You can unblock them as follows:
- Go to the phone book
- Call up the telephone book and click on the person in question so that this name appears in the blue bar.
- Tap on Options
- Tap Edit Block List.
- If there are people in this list, tap the number and you will be asked if you want to delete it; select Yes. You will then delete the number from the block list.