Good Morning
CurvalWelcome to SayNoTo0870
Finding alternative numbers can be tricky, as you have advised there are Geographic numbers for their offices.
I suspect it may divert to a specific number depending on who's covering the emergencies. Not that such a situation justifies using an 0870 number any longer.
Have you tried calling
0370 873 3878? This is a non-geographic number but is charged at geographic rates.
Alternatively it maybe worth contacting the company directly and advising you are
unable to call 0870 numbers any longer and see what they say.
Please do let us know what else you may find out, or maybe someone else here know's more than I do.