Unable to conference - only from one extension

Running FreePBX 13.0.193, Asterisk 13.18.0

I have a single extension that cannot place a 3 way call. We have about 100 Mitel 5212 phones, all the same R8. The only difference with this extension is it was one of a few originally set up as chan_sip, converted to PJ_sip. When the salesman attempts a conference he dials the first number, presses the conference button, dials the second number, and is offerered the option to transfer or “NA” The NA is in place where conf should be. I have swapped the phone, deleted the extension and re added it, with no affect. What am I missing that would affect a single extension or user? Any help would be greatly appreciated.

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