Are you suggesting it is free to call from a UK mobile, which would be welcome news to me, or that the number works from anywhere with no comment from you on cost? I would expect it to cost £1/min or thereabouts so shy away from that expectation given the long calls involved.

I have long used Skype to call for free from anywhere. Microsoft has sort of mothballed it but it still works from https://calling.web.skype.com/ if you have an account. Some other internet calling apps should work too