I've installed a CME and I have VTA on the phones, the video works great, but when I have video on, I am not able to make any calls out to the PSTN.
Once I exit the VTA application on the PC, I am able to call.
I had the exact same problem. See the link below:
It refers to CCM, but the fix is applicable to the voice port. The short version of it is, go to your voice-gateway, go to voice-port config for your PSTN trunk and enter bearer-cap speech. This should get you rolling.
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