ppp times out

I have a PC that until fairly recently could hook up to the local 
dial-up ISP under any version of Linux I tried, and now can not.  It 
still works under WinXP, but I want the better firewall safety of Linux. 
 The ISP *says* that they've changed nothing on their end, but I *know* 
that nothing changed on mine.

It worked under Mandrake 7.x, 9.1, 10.1, Mandrake move, then Fedora Core 
 2 (which is what I'm using now on that machine), using either kppp or 
wvdial.  Now, none of them work; it always times out waiting for the ppp 
daemon to start.

Here is the output of the ppp log file, using kppp:

Nov 19 09:56:39 monorail pppd[2828]: pppd 2.4.2 started by root, uid 0
Nov 19 09:56:39 monorail pppd[2828]: using channel 2
Nov 19 09:56:39 monorail pppd[2828]: Using interface ppp0
Nov 19 09:56:39 monorail pppd[2828]: Connect: ppp0 <--> /dev/ttyS0
Nov 19 09:56:39 monorail pppd[2828]: sent [LCP ConfReq id=0x1 <asyncmap 
0x0> <magic 0x7f68e1e8> <pcomp> <accomp>]
Nov 19 09:57:06 monorail last message repeated 9 times
Nov 19 09:57:09 monorail pppd[2828]: Terminating on signal 15.
Nov 19 09:57:09 monorail pppd[2828]: sent [LCP TermReq id=0x2 "User 
request"]
Nov 19 09:57:12 monorail pppd[2828]: sent [LCP TermReq id=0x3 "User 
request"]
Nov 19 09:57:15 monorail pppd[2828]: Connection terminated.
Nov 19 09:57:15 monorail pppd[2828]: Exit.

When I use minicom, the ISP asks for login and password, then sends 
multiple lines of gibberish, then times out.

When I use wvdial from the command line, I get:

Using wvdial:

root@monorail will]# wvdial
--> WvDial: Internet dialer version 1.53
--> Initializing modem.
--> Sending: ATZ
ATZ
OK
--> Sending: ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0
ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0
OK
--> Modem initialized.
--> Sending: ATDT375-2650
--> Waiting for carrier.
ATDT375-2650
CONNECT 26400/ARQ
--> Carrier detected.  Waiting for prompt.
[1c] ` [1c] `  `||||`[1c]`[1c]|```
--> Don't know what to do!  Starting pppd and hoping for the best.
--> Starting pppd at Fri Nov 18 00:44:11 2005
--> pid of pppd: 2986
--> Disconnecting at Fri Nov 18 00:44:42 2005
--> The PPP daemon has died: PPP negotiation failed (exit code = 10)
--> man pppd explains pppd error codes in more detail.


(I don't usually run as root; this is from when I'm troubleshooting.)

Any suggestions on what to try next?