Dan,
Your engineer may be correct. Each UC520 model is packaged with a different amount of DSP resources. Depending on the specific model (8/16, 24/32 user, etc) you have purchased you may not be able to configure an entire 23 channel PRI due to insufficient DSP resources.
DSP resources are shared amongst several features inside the chassis, including: hardware conferencing, MTP, and transcoding. So, the other DSP-consuming services you have configured on the device may also impact your ability to implement a PRI.
This URL displays the DSP capability in table form: https://supportforums.cisco.com/docs/DOC-9744.
This should answer any questions you may have. If not, feel free to post.
Thanks,
--Mike