All this was done as root:
When configuring a connection through the control centre, it strips alpha
characters from the phone number. I and many other people need to be able to
put "W"s in our dial strings to instruct the modem to wait for a second dial
tone and continue dialling. I didn't try putting a "," in to see what
happens. There are other times when letters are useful as part of the
number. It's a bad idea to correct peoples' "mistakes" without telling them.
I tried to configure connection sharing but it complained that there was a
problem installing iptables, I have no idea what the problem was as there
were no details given.
After it failed, it had reconfigured my eth0 to 192.168.0.1 (it did warn
that it would reconfigure but gave no details about why and how). It had
previously been on 10.0.0.9 which is also a private network, so there was no
need to reconfigure.