autodialer with no land line

["Alan" <a__l__a__n@xxxxxxxxxxx>]

I am looking for a way to set up an autodialer on my home security
system without having "land line" phone service.  Two possibilities
come to mind:

1) a device that connects the autodialer to my cell phone so that the
autodialer can call out through the cell phone network.  Down side:
requires an additional (compatible) cell phone and an additional line
on my cell phone account ($$).  I've looked briefly at the Dock N Talk
and a couple of similar devices.  All those options seem to be too
expensive for my taste (several hundred dollars for cell phone + dock
device + a year of additional line + autodialer).

2) Some sort of interface to Skype enabling the call to go out through
my cable internet service.

There may be a third possibility.  My alarm system can close (or open)
a relay when announcing an alarm.  There may be a way to use this
rather than an autodialer to communicate through a computer (triggering
a SMS message via the internet for example).  Of course a magic box and
some software would be necessary...

Has someone found a product to do something like this?  Any
suggestions?

Thanks