I finally succeeded to connect with my Samsung phone (only with pppd under xterm at this stage) by adding the command noccp (compression control protocol) to my ppp script. However, I still can't connect with the GUI tool even after turning off the ppp compression.

When I am "manually" connected I can use Manaos but not Opera since this browser thinks I am not connected (no GPRS icon appears in this case) and launches the connexion tool