Sometimes , when you defined your mgcp config and before to activate it with the CUCM config. You can experience issue where you can not send or receive calls on this gateway only because your link is not up.
This is caused because the binding between the MGCP gateway and the CUCM is not properly set up. So in order to fix this issue under your serial interface , remove the command isdn bind-l3 ccm-manager and put it again just after. The result is direct and your PSTN will go up .