Hello & Welcome to Just Answer,
Sorry I didn't see your question earlier and to hear of the issue you are having with your phones.
May I ask:
1) How many TOTAL handsets do you have with this system, and if more than one, do ALL handsets have the same issue?
2) What phone service provider (phone company) you have for this phone line?
3) If you have ANY other devices (phones, fax, alarm system, etc.) attached to this same phone line (not part of the cordless system)? If one of these other devices is a CORDED phone, does that phone have the same symptom?
4) Does this happen for ALL CALLS - both incoming and outgoing?
I ask all these questions because in my experience, symptoms like these (one side of the conversation can not be heard) is MOST OFTEN traced back to the phone service provider's service. This is especially true if the service provider is "digital-based" - such as from a cable or internet company.