The word "unterhalten" (in the sense of "to entertain / sustain / chat with sb.") is a verb with a non-separable prefix:
"Ich unterhalte mich"
NOT:"Ich halte mich unter."
Typically, these verbs form their Partizip II without the -ge- affix:
"Ich habe mich unterhalten."
Nevertheless, the Duden lists "untergehalten" as a legitimate Partizip II. I wonder how this can be true, and in which regions of the German-speaking world people would say:
"Ich habe mich mit ihm untergehalten."