From aa-asterisk.org.uk wiki
NDO numbers are "National Dialling Only".
These numbers are typically used for:
- termination points for NTS numbers
- remote communications, such as point-to-point modem communications
- remote alarm systems
- other lines not expecting incoming calls from the public.
Some providers will not connect calls to these numbers when dialled.
If outgoing calls are made from these numbers, the associated caller ID information, aka Presentation Number, must be altered so that these units display a normal geographic or non-geographic number in order for people to be able to return the call.
Example NDO numbers:
- 020 0077 3456
- 024 1155 6789
- 0116 101 2345
- 0141 011 7890
- 01707 106108
- 015242 11333
- 016977 01234