I agree with 'helpifican' in that you want to make sure that every piece of NEC equipment is at the latest version. Based on description of issue, it's most likely not SIP trunk related. I would check the following:
1. Compression setup-what is in place now?
2. QOS on the local network-Is QOS setup in the 8100?
3. Check echo cancellation settings in the 8100-check to see if the settings are all at default values.
4. Is the local network segmented- VLAN or seperate network for voice.