Asterisk.. Bad sound quality on SIP

A Asterisk is installed and a script is created that calls a SIP. The script is triggered from a Pyton page and sends a language,telefonnumber and a PIN code The telephone user is asked to enter his/her PIN and the PIN entered is validated aginst the trigger PIN. The respons back is Valid och Not valid This workflow is working but the sound quality is bad. This workflow and the soundfiles was setup on another Linux server and worked had OK sound quality The task is to help correcting the soundquality on the Asterisk/SIP connection