I'm having a similar problem on an elderly friend's Xubuntu 12.04. I've added her to the dialup group but without success.
AFAIK, there is an on-board modem. (There is no suitable connector for the only old dial-up modem I possess.) Running lshw produces
Q1. Does she have a modem?
description: Communication controller
product: V.92 56K WinModem
vendor: LSI Corporation
physical id: 5
bus info: pci@0000:03:05.0
width: 32 bits
capabilities: pm bus_master cap_list
configuration: latency=64 maxlatency=14 mingnt=252
resources: memory:cfffec00-cfffecff ioport:e000(size=8) ioport:d800(size=256)
Q2. How do I configure it?