If you are on MOL1,
read this thread. It fixed that exact issue on my phone. BTW, if you are on MOL1, think long & hard before you go to MOL2/MOL3 -- right now you have free DUN assuming you have 5 GB/month. If you do go with MOL2, you can still use DUN w/ PDAnet (to me, paying $15/month in addition to the $45 is a kick in the nuts)