I need to place a few calls to London and need to find an inexpensive/free way to do it. These are to people I have not met at business landline lumbers.

I usually use WhatsApp but my understanding is it only works for contacts already connected with cell phones.

Do any of you tech/communications experts know of an app or a way to do it? I thought Google Voice but it looks like they need to have mobile phones for that too?

Thank you in advance.