Why can I not hear incoming voices on my calls and the caller can't hear me ??

[Taz19560]

My phone rings, I answer, the caller cannot hear me and I cannot hear them. If however I use Viber or WhatsApp (ie internet calling) the phone works correctly and perfectly on speech and sound. I have put my SIM in another phone and all is good. I have spoken with my serviceprovider and they have checked my service, so i assume the fault lies somewhere within my 4s ?? I can receive text messages. I just need to understand why the phone works over the internet but not through my provider. PLEASE HELP.....

 

[Bigeric23]

Does the speakerphone work?