[CentOS] modem for dial IN

Hi

Not entirely centos related but that what the boxes are running so here i
go ;)

2 of our centos boxes run nagios for network monitoring and send email and
paging alerts and are connected to standard phone lines using modems to
send the pages. I'm looking for a way in the event of provider failure to
login to these machines and so am looking at the ability to connect to the
console over the modem.

Any good how-to's on this ? Also interested to know real life seciruty
risks etc. Is it possible to limit phone numbers that are abor to connect
etc?

thanks