HELP! Packet data connection not available!

Hey, I was wondering if anyone knows how I could get internet working
on my Nokia. Everytime I try to access the internet on the phone's
browser, I get a red stop sign and a "packet data connection not
available" message. It is a Nokia 6126 and my service provider is
Dobson Cellular One. I've already tried downloading the GSM settings
and Text messaging settings from the Nokia website, but it still
doesn't work. I've also changed the "Packet Data Connection" option
to "Always Online" under settings > connectivity, but it still doesn't
work.
The weird thing is that when I switch the phone off and turn it back
on, the internet works for around two minutes until I get the "Packer
Data Connection Not Available" message again. Sometimes the "Link
Failed" message comes up too after that message. I have full service
when I try to access the internet, however. Help would be
appreciated.

Thanks in advance,
Krupa