Headset Buddy models 01-PC35-RJ9A and 01-PH25-RJ9A with RJ9/RJ10/RJ22 plugs should work with your Aastra phone. You may have to modify the audio and microphone settings on your phone to make it work properly.  Here are some troubleshooting steps to try:

1. Try changing the microphone gain setting to "High" or "Low". Find more information on how to do this in the phone manual.
2. Try changing the switch at base back to different positions. For example, try positions 1 or 7 as this will increase microphone sensitivity.