There is no complaint on the sound quality. For 3-way calls, i had tried one external and two internal only without any sound quality issue.
The model is 501-1204-RP. FYI, the main reason for me to try Quintum is to resolve drop call issue.
Also, you may be able to improve the sound quality by changing the speed to 30ms instead of the default 20ms. But you may notice a little delay.