Cellphone not Charging after using it for 3 weeks

I bought Sony M4 Aqua and it was delivered to me. used the phone for 2-3 weeks now and the phone is not charging anymore. the phone did not fall or get damaged in anyway. When I try to call FNB I got transferred to the Insurance line and no one picks up. the phone just keep ringing. in the mean time I have a phone I cant even use since the battery is dead. So Annoyed!!! Don't even know where I can take this phone so it can get fixed..... FNB has so many issues lately its so annoying. On weekend I couldn't even buy bundles from celphone banking. Now This!!! I haven't even started paying the instalment and already the phone is giving me problems......