I am managing a PBX which runs redhat enterprise linux. The time on it is always getting out of sync, which causes the time on the phones to display incorrectly.

I can't figure out for the life of me why the time keeps changing. How can I set it to update it's time every day or two? I don't want to hit ntp.org every day, as is recommended. What's the recommended interval for connecting to the ntp pool?

The kernel is Linux 2.6.9-67.0.4 (from uname -a)

The recommended frequency to hit a time server is once. It's good
to do that immediately before enabling the ntp service to run.

It's rather easy to set up ntp in RHEL. Run "yum install ntp"
then "ntpdate time.nist.gov". To synchronize the hardware
clock, run "hwclock --systohc". Run "chkconfig ntpd on" to
ensure that ntp starts at boot and run "service ntpd start"
to make it start without rebooting.

Of course you can use another server instead of time.nist.gov
or you can edit /etc/ntp.conf if you don't want to use the
default servers.