Damian,
From the chat output it looks like the radio is responding. The problem may still be radio related, but I am not sure. You can turn up the ppp debugging by:
Diagnostics
Messages are sent to the syslog daemon using facility LOG_DAEMON. (This can be overridden by recompiling pppd with the macro LOG_PPP defined as the desired facility.) See the syslog(8) documentation for details of where the syslog daemon will write the messages. On most systems, the syslog daemon uses the /etc/syslog.conf file to specify the destination(s) for syslog messages. You may need to edit that file to suit.
The debug option causes the contents of all control packets sent or received to be logged, that is, all LCP, PAP, CHAP, EAP, or IPCP packets. This can be useful if the PPP negotiation does not succeed or if authentication fails. If debugging is enabled at compile time, the debug option also causes other debugging messages to be logged.
Debugging can also be enabled or disabled by sending a SIGUSR1 signal to the pppd process. This signal acts as a toggle.
You can either send a SIGUSR1 to the pppd process or add the debug option to the ppp start script. It would probably be easier to try the SIGUSR1 first.
Jeff