After adding UDP/TCP ports and putting the MicroCell on a dedicated IP, I decided to try one other thing:

I enabled IPv6 mode (tunnel mode), left incoming connections blocked, configured it automatically, and allowed IPSec authentication in my IPv6 firewall on my Apple AirPort Extreme.

This seems to have done the trick.  After rebooting the modem, router, and MicroCell, I can make calls the past few days with only about a 5 second delay.

Here's hoping the problem is fixed!