in /etc/modprobe.d/options
options hisax type=N

use ippp1. ippp0 should be used for dialout, as it sets the default-gateway.
device.ippp1

LOCALIP=10.0.1.1
REMOTEIP=10.0.1.2

LOCALMSN='12345678'
REMOTEMSN=''
#LEADINGZERO=0

DIALMODE=auto

# dialin from everywhere
isdnctrl secure ${device} off

ipppd.ippp1 name XXX # chap

lcp-restart 1           # Set timeout for LCP
login
auth
+pap
+chap                   # set this to -chap id you want only pap
noccp                   # compression negotation on/off
nolzs                   # LZS compression off (not yet supported fully)
noipdefault             #  do not  use name for default ip addr
useifip         # (do)     use ip addresses from interface
netmask 255.255.255.255 #  do      set netmask, 255.255.255.255 for pointopoint
nohostroute             #  do not  set host route
mru 1524                #  do      set maximum size of receive units to X
mtu 1500                #  do      set max size of transmit units (1500 is OK)
ms-dns 10.0.1.1         # (do not) set dns addr for the peers use (dialin only)
noipx                   # en/disable ipx

only if you want chap /etc/ppp/chap-secrets and /etc/ppp/pap-secrets

"dialinuser" "*" "dialinpasswd"

for pap using systemusers: /etc/ppp/pap-secrets

* * '' *

mgetty in inittab

I0:2345:respawn:/sbin/mgetty -D -m '"" ATZ OK AT&E12345678 OK AT&B512 OK' -s 38400 ttyI0

enable forwarding:

echo "1"  > /proc/sys/net/ipv4/ip_forward

you mays need or want other stuff like rp_filter etc

dns cache using pdnsd ...
/etc/pdnsd.conf if you are not using resolvconf, configure the server-part:

server {
        label="server0";
        ip="192.168.0.254";
        timeout=30;
        interval=30;
        uptest=ping;
        ping_timeout=50;
        purge_cache=off;
}

use 'pdnsd-ctl status' and dig to test your settings