Assuming you are using DDR here with dialer profiles. You can stop the NTP updates from triggering the calls by denying the NTP traffic in your dialer-list which will define "interesting traffic" that can make a call be made.
The following defines NTP in RFC1700 (I am unsure if your SNTP uses the same)
ntp 123/tcp Network Time Protocol
ntp 123/udp Network Time Protocol
dialer-list 1 list (ACL#) (you'd use this with an ACL denying NTP and permitting what you want to trigger a call)
!-- Interesting traffic is defined by dialer-list 1.
Here is a URL with the basic's of dialer-list command:
http://www.cisco.com/en/US/partner/products/sw/iosswrel/ps1828/products_command_summary_chapter09186a00800ed40c.html
This would not allow NTP to trigger a call but would allow NTP to flow after the call was established. Hope this helps.
Don
