Hello, Perhaps this helps, via 192.com: 028 9023 4070.  Please Post your result.
Rgs, Rod

Sorry mate! I did`nt dial as a test before i posted for you(most times i do but i thought i`d cracked it with this one)If Lewis Group have tied this up tight then its a task for the Mod`s, but i`ll keep searching.
Rgs Rod.

There is one verified database entry for Lewis Group which is for 0870 7513300 and gives 01274 852000 as an alternative.

There are two other unverified entries which give the same 01274 number: 0870 0567800 and 0870 7513387. The latter looks similar to the other 0870s quoted. Do they have a range of 0870s that go to different members of staff or are the 0870 number individual to their client companies?

The OP says that the website gives 0870 751 3401, but it now gives 0870 751 3300. See this page cached by Google on 24 May.

The Lewis Group Limited Registered in Scotland No: SC127043 Rowan House, 70 Buchanan Street, Glasgow, G1 3JF
From 192
0141 567 7300
0141 567 7400 (fax)

Now I know you wanted Belfast, but try this theory.

0870 7513 300 is the main number listed for them
0141 5677 300 A similarity.

Some companies keep 0870 numbers that are similar to the true number, thus making it easier to ring inter-branch.
Perhaps a play with the Belfast number might work.