It's been working since 2004 in a couple of different 
systems.  In the first, it wasn't so kernel-based (as 
far as I remember) and had configuration of the /etc/ppp/*
files (perhaps via rp-pppoe?, even though it's over ATM?).
In the second it had its settings in the gentoo startup
conf.d scripts, but seemed less reliable, losing the 
connection every few weeks without recovering it.
Now to see if a new setup on a new computer is any different.

/etc/conf.d/net :  
	config_ppp0=( 'ppp' )
	link_ppp0='0.38'
	plugins_ppp0=( 'pppoa' )
	pppd_ppp0=( debug updetach noauth debug defaultroute maxfail 0
	            noaccomp nobsdcomp noccp nodeflate nopcomp novj novjccomp
	            persist child-timeout 60 )
	username_ppp0='.....@oneteldsl3.net'
	password_ppp0='.......'

emerge net-dialup/speedtouch-usb
(installs the following firmware:
	/lib/firmware/speedtch-{1,2}.bin.{0,1,2,3,4}
	(only 4 actual files: 0,1,2 are same, 3,4 are same)

cd /etc/init.d && ln -sv net.lo net.ppp0

/etc/conf.d/local.start
( echo "----------------"; date ) >>/var/log/ppp0_speedtch.log
/etc/init.d/net.ppp0 start >> /var/log/ppp0_speedtch.log 2>&1 &

and add further to /etc/conf.d/net a post-up function:
	function postup() {
        if [ "$1" == "ppp0" ] ; then
                /etc/scripts/ipfilter.sh start
                /etc/init.d/ntpd stop
                /etc/init.d/ntp-client restart
                /etc/init.d/ntpd start
        fi
}


All running ok immediately.  Yet to see about reconnect-reliability.