What IOS version are you using? If this is in the first few seconds of the call, then see below:
CSCdv66278
Description: Echo occurs on VG200 for digital T1 calls during the first few seconds of the call.
Workaround: This problem has been fixed in Cisco IOS Release 12.1(5)XM7.
If not, if it is random or constantly during entire conversation, go through step by step troubleshooting, the first of which is make sure that the PRI DS1 is running absolutely clean and error free at all times.
http://www.cisco.com/univercd/cc/td/doc/cisintwk/intsolns/voipsol/ea_isd.htm
Hope this helps.
