N.b. if that works and it connects you (it will work in some cases) there is NO guarantee that you won't be charged by your phone company. And if you are charged, it will probably be international rates.
While it is nice that it works, I think it is reasonable to pay for such a call.

True international toll free numbers are +800, i.e. 800 as an effective IDD 'country' code, e.g. 00800 from countries where 00 is the local IDD access code.

But int+800 is only supported from some phone networks in some countries, and obvioulsy, the company you are calling needs a such a international freefone ++800 number. [I.e. obviously you can't just dial a +44 (0)800 number as +800]