I recently turned up a new Webex Calling system for a customer and it is going through an MX67.
I made zero changes to the MX firewall rules and it works like a champ.
I used the default traffic shaping rules and set the WAN1 interface bandwidth to conform with
the speed of the single fiber internet connection. I also had to set QOS to trust the DSCP values
from the phones on the Voice VLAN of the MS120-48FP switch.
I agree with BrandonS that you should take a look to see if their are any port forwarding rules
or firewall rules that could be interfering with the functioning of the system.