You might try using dty00 instead.  This is the dialout device, which
doesn't required CD (carrier detect).  But I suspect that's not it.

Why do you list 'noauth'?
Look at the chat debug logs, and enable pppd debugging.  It may be
that authentication is failing and the ISP is hanging up on you.

tcpdump would be interesting.  You might have to ifconfig ppp0 up and
start it before dialing - tcpdump won't start on down interfaces, but
once attached will print out packets whenever it is up.

-- 
        Greg Troxel <gdt@ir.bbn.com>