Recommended Codec - XtenSoftPhone 2.0 GNU/Linux

Hi,
I'm running a T1 PRI, sometimes I see congestion messages in the console and I feel like it's due to the line running out of bandwidth. What is the best codec to use that sounds the best and uses the least amount of bandwidth possible?
I'm running a T1 PRI, sometimes I see congestion messages in the console and I feel like it's due to the line running out of bandwidth. What is the best codec to use that sounds the best and uses the least amount of bandwidth possible?