I bought matrix SIM for my travel in Turkey. Initially I was told that incoming call would be free but when bill came, they were charged. Before I could take up the matter with customer care and find a resolution, my credit card was charged, of course with charges of incoming calls. Again, they do not say that SIM they provide is from UK. So when you are in a country, other than UK, you will be charged roaming charges. In general, I found Matrix cellular company unreliable and unethical.
I advise not to use their products, or when you use them, use with extreme care.