Standard bank mobile

I changed service providers from MTN to Standard bank mobile. 1. For some reason every now and then I don't receive calls. I get an sms to say that I have a missed call, but never received that call. I need my phone for business and if people can't get hold of me, I loose potential business. I have sent emails enquiring as to why this happens, but to date I haven't received a reply. 2. I want to use the phone while travelling out of the country. I need to pay a deposit to activate this feature. I was with MTN for many years without having to pay a deposit. I understand that my calls will be charged to my account at the rate of the new country, but why a deposit. When mentioning this to SBSA, they said I should buy a sim for the country I am in. That doesn't help me at all as I travel to many countries as part of my work.