Incorrection charges on International Roaming

I traveled to USA during week of 28 August 2017 to 1 September 2017, according to MTN website and SMS I received I was supposed to have been billed R1.25 per SMS, but instead I have been billed R10 per SMS I Send 17 SMS and was therefor charged R170 instead of R21.25. I was roaming on AT&T when these charges happened