Mitel 5220 Sound issue

Hello,

I have a Mitel 5220 and I can hold a two-way conversation on the speakerphone but when I pick up the handset I can still hear but the other party can not hear me. I have tried another handset and I have tried another cord. What else could it be? Several have suggested a codec issue but why would the speakerphone work if that was the case?

Thanks,
Nicholas