I think you'll find that's
cating the current content of /var/log/syslog and so then the
tail -f has nothing new to follow.
Try
tail -f /var/log/messages | some grep r.ex. here
I used a Speedtouch 330 USB ADSL modem for years, but now we've got a router, so I can't easily see exactly how the relevant log lines start to try come up with the filtering command.
IIRC there's been several different ways to use that modem, various kernel and userspace modules and utils. The initial part to load the firmware into the modem and sync the line can be configured to be more verbose, and the subsequent
pppd call adsl gives regular pppd log entries about the connection being up or down.