If you would like your softphone to ring out the big speakers but your caller's voice to come in through the headset, that might be possible on a Mac.
Here are two things you can try:
1) In the Audio/Video settings for your softphone try setting your Ringer to the built-in default speakers. Leave the Microphone and Speakers set to the headset.
2) If that doesn't work or isn’t an option, try going into your System Preferences. Set up your Output sound as the Internal Speakers. Leave your input sound as the headset and leave your softphone settings pointing to the headset.
This may or may not work depending on what type of softphone you are using. But it’s worth a try, as you can always put the settings back!