You doomed yourself by the thread title, this isn't so much a pennytel issue as a device handling the fail over, if you want a simple solution there is voxalot, alternatively if you have an old PC lying about you could always setup your own asterisk server there is a number of cd installers that handle everything for you such as
PBX in a Flash. Lastly the final option would be something like a WRT54G loaded with OpenWRT and then load asterisk on it, although this isn't for the faint of heart but I'm sure someone would have put up a step by step how to somewhere on it.