[xubuntu] OpenDNS settings for pppoe with usb adsl modem

hi to all,I have an old speedtouch usb modem (speedtouch 330 revision 0) and on my desktop with xubuntu 12.04 I've configured a pppoe connection. I can connect and I see that my ISP assign an IP address and the DNS (I don't know why the primary DNS address is not reachable by ping while the secondary yes) but no address is resolved then I can't surf the web. Then I want to set the open DNS but there is no way, if I change manually /etc/resolv.conf it is rewrited by some script (there is the flag usepeerdns on the configuration script, if I exclude it there is no way to assign any DNS server because for some reason /etc/resolv.conf is not read, I think) also if I set not writable the file changing the default permission. I changed dhclient.conf with the code

Code:

prepend domain-name-servers 208.67.222.222,208.67.220.220;

and now if I connect by a wifi connection to my router I'm using openDNS server but ppp does not use this script as long as I can see and the DNS server is always setted by my ISP. How can I use set DNS manually to a PPP connection? Is there any way to change it after the connection? Why NetworkManager is not able to manage my dsl connection, it seems not able to manage the dsl usb cable modem? If I use pppoeconf NetworkManager doesn't start and I have to manually delete the lines added to /etc/network/interfaces because the system is not able to start with full configuration of network If I connect a modem-router to the same line I can surf with the DNS server assigned by my ISP (the primary is not pingable, also in this case), I can't figure why. Some suggestion? Thanks to all