/etc/ppp/ppp.conf
default:
set log Phase Chat LCP IPCP CCP tun command
ident user-ppp VERSION (built COMPILATIONDATE)

# Ensure that "device" references the correct serial port
# for your modem. (cuaa0 = COM1, cuaa1 = COM2)
#
set device /dev/ucom0  #motorola c380-USB!!

set speed 115200
set dial "ABORT BUSY ABORT NO\\sCARRIER TIMEOUT 5 \
           \"\" AT OK-AT-OK ATE1Q0 OK AT+CGDCONT=1,\"IP\",\"internet.volga\" OK
\\dATDT\\T TIMEOUT 40 CONNECT"  #modem initialisation string AT+CGDCONT=1,IP,"internet.volga" Megafon Povolzhe-Ulyanovsk
set timeout 60                 # 3 minute idle timer (the default)
enable dns                             # request DNS info (for resolv.conf)

papchap:
#
# edit the next three lines and replace the items in caps with
# the values which have been assigned by your ISP.
#

set phone "*99#"
set authname p
set authkey n

set ifaddr 10.0.0.1/0 10.0.0.2/0 255.255.255.0 0.0.0.0

/etc/ppp/ppp.linkup
papchap:
  add default HISADDR

запускаю так:
[root:ns:/usr/home/rippy]>ppp -nat papchap
Working in interactive mode
Using interface: tun0
ppp ON ns> dial

(нат - потому как выступает в роли гейта машина)