This could be a couple of things.
Let start with the easy things first:
Reboot your 3300.Next do a couple of test: Make ~10 calls into one of your own DID. See if you get any failures. If not make 20 more just to be sure.
Then call somewhere else. Could be a cell or someones home. ~10 calls.
Then call someone long distance. ~10 calls.
What you're looking for is to see if you can narrow down failures to LD calls, local calls or calls within your own CO.
Next, have your carrier work with you to trace a failed call. Too often when you report a problem to a carrier they will just check the T1 for error. That's not the problem here. You need to specifically request to work with a switch tech to do call traces to isolate the cause of the one way audio. They will monitor while you make enough calls to create the failure and then they can more closely ID the cause. It may be something as simple as a protocol mismatch.
Although it is possible that you may have a problem with a DSP, it wouldn't be my 1st choice. A reboot may resolve that in any case.
Also, as martyn suggested, you can turn on traces in the PBX.
The command is 'ccs trace en cont'.
Make a phone call and dupe the issue.
To turn off the trace 'ccs tra dis'
You can post it here if you like.
I'm not an expert at reading the traces but there some people here I think can.
Ralph