voip phone to landline communications ve

we have cisco 7960 phones remotely connected using T1’s in 4 remote sights comming back into a cisco 3640 Gateway at a central location.
Each remote sight has 1750 Cisco routers where the T1 ends.
We have callManager3.1 & Unity
PROBLEM:
When a user calls from a landline to any VOIP phone.The caller can’t hear the voice properly but the receiver can hear it properly.
What can we do to fix this problem
Internally in our network an ipphone to a landline works just fine but when we call outside our network the receiver will here very very low voice.