the general EMEA support line, 00800 2222 7172, was free from my o2 mobile on contract (I have a volt contract which has worldwide roaming), it was very difficult to find a definitive answer online so I just tried it and wasnt charged. This was a few months ago.

Just checked my bill and it was listed as "International calls - UK to overseas 25 mins £0.00"

If you have inclusive minutes, use the 44 1950 499397 one as that is a UK landline