New clue! Lancaster changed from 0870 to 0844 this year or last year. The provider, Uni-Tel, provides a page where they will translate from the old number into the new one.
https://www.unitel.co.uk/phone/index.php/number_change.phpA previous poster mentioned that numbers were in the form 0870 754 21xx.
I've just tried some numbers:
0870 754 2100 no such number
0870 754 2198 -> 0844 308 5426
0870 754 2199 -> 0844 308 5427
0870 754 2200 -> 0844 308 5428
There's an obvious pattern there!
Another range mentioned right at the start of the thread:
0870 753 5567 -> 0844 308 5037
0870 753 5597 -> 0844 308 5067
This may be a clue to how the new numbers map to the old 0870 numbers, and the 0870 numbers might relate more clearly to geographical numbers.
When I have time this evening, I'll try lots more numbers to see if I can find the equivalent number to the one Bens_Mum is after:
0844 307 62xx
Or perhaps someone else might like to try before then!