I though it only happens in analogue but unfotunatly it happens in IP telephony world.
i have a 2811 router, whcih is Voice gateway, SRST, CUE and transcoder.
sometimes a problem happens when some one from an IP phone (internal) calls a person on the outside (PSTN), the IP phone do not hear anything and suddenly he is with a conversation with a guy who called in to the company and dialed another extension!!!!
it happened more than once, i have noticed a cpu usage between 30 to 50 average of the VG router.