Your issue with parking phone lines is not relevant to the OP, so would be distracting to comment on.
Other posters have suggested Skype, which is correct, but only obvious to a seasoned traveller.
I have installed Skype on my phone and literally the only thing I use it for is phoning BA, either when abroad or when I need to ring a non-UK call centre out of hours.
And I probably wouldn't have even done that if I hadn't heard people talking about it on FT.