That is correct, your PABX gets the clock from PSTN, and your VG gets the clock (external) from your PABX. This way all devices are synched to the same clock source.
Call Manager VG > PABX > PSTN > ... > Global Master Clock
You can change your clock source to external anytime. It won't cause any outage because the active clock will cut-over only after the VG will synch with the external clock. It is a smooth process, and it will complete in a couple of seconds transparently.
As I said in my previous post Clock slips may cause your fax and modem (POS devices) fail. Voice calls/quality are not affected by clock slips.
