We tested this again on our latest version and it seemed to have worked.
We made an inbound call to our auto attendant, an ext picked up via user portal, put this call on hold, made another GSM call, hit the conference button on the user portal, all 3 of the participants heard the conference tone and has 2 way audio on all channels.
Are you able to successfully do this between 3 extensions on the PBX? Maybe your trunk is hitting a limit or not working as expected. This is all assuming that your ad-hoc conf room was created and is working as expected.