Aha... SOME monthly plans get the 150 calls free, some don't.
From the t-mobile website
Quote:To call Customer Services from your T-Mobile phone, simply dial 150 (Standard Pay monthly, U-Fix & Mix It plans - Free; Pay as you go - 25p per call; Online/Self Serve plans - 50p per minute (Max £5 per call))
I have the online/self serve option (which saves money as long as I never need to phone them)