If you use dialer pool's you can only terminate one call per dialer interface and that is why you are having the issue. If you want to terminate more than one call you would need to configure more dialer interfaces.
But to make things easier you can just change the serial interface to use a rotary group instead of dialer pool.
dialer rotary-group x (where x is the dialer interface number)
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...