Now that you have sorted out the serial port and modem settings it is a good idea to make sure that these setting do indeed work by dialing you ISP and seeing if you can connect.
Using you terminal communications package (such as minicom), set up the modem initialisation required for PPP and dial into the PPP server you want to connect to with a PPP session.
(Note: at this stage we are NOT trying to make a PPP connection - just establishing that we have the right phone number and also to find out exactly what the server sends to us in order to get logged in and start PPP).
During this process, either capture (log to a file) the entire login process or carefully (very carefully) write down exactly what prompts the server gives to let you know it is time to enter your user name and password (and any other commands needed to establish the PPP connection).
If your server uses PAP, you should not see a login prompt, but should instead see the (text representation) of the link control protocol (which looks like garbage) starting on your screen.
A few words of warning:-
some servers are quite intelligent: you can log in using text based user name/passwords OR using PAP. So if your ISP or corporate site uses PAP but you do not see the garbage start up immediately, this may not mean you have done something wrong.
some servers require you to enter some text initially and then start a standard PAP sequence.
Some PPP servers are passive - that is they simply sit there sending nothing until the client that is dialing in sends them a valid lcp packet. If the ppp server you are connecting to operates in passive mode, you will never see the garbage!
Some servers do not start PPP until you press ENTER - so it is worth trying this if you correctly log in and do not see the garbage!
It is worth dialing in at least twice - some servers change their prompts (e.g. with the time!) every time you log in. The two critical prompts your Linux box needs to be able to identify every time you dial in are:-
the prompt that requests you to enter your user name;
the prompt that requests you to enter your password;
If you have to issue a command to start PPP on the server, you will also need to find out the prompt the server gives you once you are logged in to tell you that you can now enter the command to start ppp.
If your server automatically starts PPP, once you have logged in, you will start to see garbage on your screen - this is the PPP server sending your machine information to start up and configure the PPP connection.
This should look something like this :-
~y}#.!}!}!} }8}!}$}%U}"}&} } } } }%}& ...}'}"}(}"} .~~y}
(and it just keeps on coming!)
On some systems PPP must be explicitly started on the server. This is usually because the server has been set up to allow PPP logins and shell logins using the same user name/password pair. If this is the case, issue this command once you have logged in. Again, you will see the garbage as the server end of the PPP connection starts up.
If you do not see this immediately after connecting (and logging in and starting the PPP server if required), press Enter to see if this starts the PPP server.
At this point, you can hang up your modem (usually, type +++ quickly and then issue the ATHO command once your modem responds with OK).
If you can't get your modem to work, read your modem manual, the man pages for your communications software and the Serial HOWTO! Once you have this sorted out, carry on as above.