What soft phone are you using? Express Talk and most other soft phones have a gain (volume) adjustment for both audio in & out. If you are using an external USB phone that could also be the source of the problem.
If you're using Express Talk, run the Audio Set-up Wizard and see what the mic levels are. If they're low, it would indicate either a problem with the microphone/connection or even more likely the Operating System's settings for mic volume (Note that Windows also has a feature called "Microphone Boost" that lets you really turn the gain up on the mic's input significantly (can cause clipping & hissing though)). If the volume levels appear to be normal, it does appear to be a problem in the transmission.
Even if it is a problem in the transmission of voice data, I would think the problem could still be solved to a reasonable satisfaction by maxing the gain-out on the soft phone, turning up the mic volume in the sound mixer, and if necessary, enabling the mic boost option.
I thought I should also note that the Linux version is pretty new (as in beginning of this month) and I'm not sure how many people are actually using it in real world applications as of yet. It's quite possible that this is a problem exclusive to the Linux version and has not been reported yet.