A few questions to help figure out what's happening here.
Do you see the Public Mobile name at the top of your phone?
Which plan do you have?
Have you logged in to see if your account is active? ( in other words, did your last invoice get paid)
Let us help you figure this out before you go to a CS Agent. We may be able to solve it for you.
new account or old account ?
Try to reboot the phone first, check if the phone actually connected to PM network. Then try to make calls
If still unable to do so, try Reset all networks. You can also try your sim card in another phone to rule out device problem
Last, if you cannot get it fixed, submit a ticket with CS agent:
Can’t call-out with phone
Do the other portions of your Public Mobile service work? First, let's try the obvious things (that you might have already tried) by restarting your device. Also, if you're not already doing this, try dialing a 1 before the area code when attempting to make your next call.