Up for your consideration is a KX-TDA5166 8 channel Echo Cancel card for the Panasonic KX-TDA50 Phone Systems. These are a must use item for Conference Calls and Voice traffic over IP Trunks. The card is designed to balance the voice channels and basically clean up both ends so you have a clean and clear call without feedback. The Card supports up to 8 calls at the same time with a mix of Extension, Loop CO (Pots lines) or IP Trunk Lines.
eBay