!
Access-list 101 permit udp any any eq ntp time-range sntp-dial
access-list 101 deny udp any any eq ntp
access-list 101 permit ip any any
dialer-list 1 protocol ip list 101
!
time-range sntp-dial
absolute end 00:00 01 January 2000
!
The time there doesn't really matter as long as it is later than the epoch time for the device in question, and earlier than the current time. 01/01/2000 was just the arbitrary choice I made last time I configured that.
With that config, NTP will bring up the line if and only if the clock on the Cisco has not already been set.
For an unattended installation which may not dial up very frequently,
it may be worth using a time-range which allows dialling once per day to keep the clock reasonably well synced. If your usage pattern results in the line coming up frequently, that is an unnecessary step. ConstrUCting an appropriate time-range statement is left as an exercise for the reader.
If it's a small single user LAN, it's considered polite to avoid the stratum-1 servers. Most ISPs should provide NTP servers for customer use, eg try ntp..net,timehost..net, ntp0..net,
ntp1..net, etc. Apart from not overloading valuable global resources, using a NTP server local to your ISP will probably provide a more stable time service due to lower latency between the client and server.