For EVM-HD-8FXS/DID, adjacent ports 0/1, 2/3, 4/5, and 6/7 share the same impedance-coefficient settings within each pair. This pairing is especially important when you are configuring some ports for DID mode and others for FXS mode. DID installations may require different impedance selections resulting from off-premises loop characteristics.
If you change an impedance setting, a message alerts you to the change.
These impedance settings apply to the baseboard (EVM-HD-8FXS/DID) only?not to EM-HDA-8FXS. Setting the impedance on the EM-HDA-8FXS changes only the impedance for the port being configured.
This URL should help you:
http://www.cisco.com/en/US/products/sw/iosswrel/ps5207/products_feature_guide09186a00802c4b09.html