Account overruns limit without warning

I recently travelled to Mozambique from South Africa. My MTN account is capped at R1200.00.<br> Before I entered Mozambique, I phoned the MTN call centre to confirm that my service will work, and that my cap will be effective. The call centre confirmed both (in other words, my account can't go above R1200).<br> Seven days into my holiday, MTN sends me an sms that my account has reached R5200 and will be suspended.<br> After a huge struggle, I got hold of the MTN contact Centre (because 808 does not work from Mozambique).<br> They then explained, that somehow (and they can't explain why), my account has overrun its cap of R1200.00 and I am liable for the difference. Had I known that, I would not have accpeted all the calls!<br> <br> Is it legal to let an account that is deliberately capped overrun to such an extent?