Cell C
Cell C not working properly

Hi, I changed to Cell C recently and I now have many many dropped calls.
Often people cannot get through to me just straight to voice mail.
And my what'sapp hardly works.
I went overseas and was on roaming sms and calls. And since I have been back what'sapp is even worse.
And emails hardly get through on my phone at all (maybe once a day). How do I sort this out??