A PVDM is not mandatory, but that doesn't mean a particular scenario will not need one. A reputable consultant or a Cisco partner with voice specializations should have been tasked with the design of the system, and then they would take into account the needs based on your requirements to determine whether a PVDM would be needed or not.
If you're using different codecs for example, that's a case in which you would need a PVDM as software based conferencing is limited to G711. A HW conference using PVDM can mix multiple codecs for a conference.
I would start by making sure conferencing was properly configured, and review the limitations of software based conferencing to make sure those actually meet your needs. Refer to the CME administration guide for all the configuration and details on conferencing.
If it's properly configured, then review debugs for that call in which you're trying to conference an external party to understand what happens with the call, and maybe logs from the device as the next step.
HTH
java
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