Gets the GdbusModemVoice instance for the D-Bus interface <link linkend="gdbus-interface-org-freedesktop-ModemManager1-Modem-Voice.
top_of_page">org.freedesktop.ModemManager1.Modem.Voice</link> on this, if any.
| this |
A GdbusObject. |
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A GdbusModemVoice that must be freed with unref or null if this does not implement the interface. |