The table below was compiled using information simplified from British Telecom publicity
No responsibility is accepted for any errors or omissions.
For the full explanation visit the "Big Number Change" website.
The new system abolishes the old 5/6 figure prefix for each exchange and replaces it with an NI regional code and a  two-figure exchange prefix (details below).  The subscriber's 5/6 figure number remains the same.  From the number of vacant codes in the table below, it can be seen that this new system creates the required capacity for future expansion.  The structure utilised by British Telecom for this eight-figure number is as follows:
First digit The six counties were allocated 2,3,4,6,7 & 8 alphabetically 
with 9 used for the Belfast District
Second digit Represents the town/exchange (occasionally 2nd & 3rd digit - see BT website)
Remaining 6 digits The current subscriber's number (usually 6 digits - see BT website)
Dialling within Northern Ireland - see below...
Dialling into N. Ireland from Great Britain - Use the code (028) then...
Dialling into N. I. from the Republic of Ireland - Use the code (048), then...
CODE & COUNTY CODE & AREA
   
0 NOT USED
1 NOT USED
2 ANTRIM
excluding the Belfast District, thus:		 20 Ballycastle
21 Martinstown
22
23
24
25 Ballymena
26
27 Ballymoney
28 Larne
29 Kilrea
3 ARMAGH
thus:		 30 Newry
31
32
33
34
35
36
37 Armagh
38 Portadown
39
4 DOWN
thus:		 40 Banbridge
41 Rostrevor
42 Kircubbin
43 Newcastle
44 Downpatrick
45
46
47
48
49
5 NOT USED

 
CODE & COUNTY CODE & AREA
   
6 FERMANAGH
thus:		 60
61
62
63
64
65
66 Enniskillen
67 Lisnaskea
68 Kesh
69
7 LONDON
DERRY
thus:		 70 Coleraine
71 Londonderry
72
73
74
75
76
77 Limavady
78
79 Magherafelt
8 TYRONE
thus:		 80 Carrickmore
81 Newtownstewart
82 Omagh
83
84
85 Ballygawley
86 Cookstown
87 Dungannon
88
89 Fivemiletown
9 BELFAST
DISTRICT
thus:		 90 Belfast
91 Bangor
92 Lisburn
93 Ballyclare
94 Antrim
95
96
97 Saintfield
98
99
The dialling situation within Northern Ireland is somewhat complicated.  There are two changes:
22nd April 2000 Local calls The 5/6 figure local numbers are replaced by the new 8 figure number - 
eg (Enniskillen) 123456 becomes 6612 3456
16th September 2000 Trunk calls Until this date either the old 11 figure number
or the new 8 figure number prefixed by 028 can be used.
After this date the new 8 figure number must be used on its own -
eg 01232 123456 becomes 9012 3456