You can test the settings by manually establishing a PPP connection. (Once we have the manual connection working, we will automate the process).
To do this, your communications software must be capable of quitting WITHOUT resetting the modem.
Minicom can do this - ALT Q (or in older version of minicom CTRL A Q)
Make sure you are logged in as root.
Fire up you communications software, (such as minicom), dial into the PPP server and log in as normal.
If you need to issue a command to start up PPP on the server, do so. You will now see the garbage you saw before.
If you are using pap or chap, then merely connecting to the remote system should start ppp on the remote and you will see the garbage without logging in, (although this may not happen for some servers - try pressing Enter and see if the garbage starts up).
Now quit the communications software without resetting the modem (ALT Q or CTL A Q in minicom) and at the Linux prompt (as root) type...
pppd -d -detach /dev/ttySx 38400 &
The -d option turns on debugging - the ppp connection start up conversation will be logged to your system log - which is useful if you are having trouble.
Your modem lights should now flash as the PPP connection is established. It will take a short while for the PPP connection to be made.
At this point you can look at the PPP interface, by issuing the command:-