I need a suggestion for phone line interface to computer

At present I have a Dell workstation 5820. I am using a Creative Labs Soundblaster Z card. I am trying, mostly unsuccessfully to make it interface properly with my Plantronics MX10 headset amplifier. I cannot seem to get enough output level from the MX10 to bring the recording level with Audacity up to normal. I have tried many settings. Perhaps the solution lies in a dedicated interface with the analog copper pair on one side and several inputs and outputs available such as audio in and out. USB connectivity and whatever else that might make life easier. That way I could simply use the Plantronics MX10 for the Supra binaural headset that I use alone and have the new device to the other chores. I need 1. To be able to record calls naturally  and; 2. Be able to play output from my computer over the phone line. There are SO many devices that claim the world but I have no idea which one to choose to try. I would ONLY buy it from Amazon since returning an unsatisfactory product is easy. I do not need call time and date logging. I actually MIGHT have everything I require at this time but making it all work together has proven elusive to say the least. The present phone system I have is an Avaya Partner system with 18D desksets. The MX10 connects between the deskset and the handset in series.