ACD call to ring an IP phone and a DECT phone in parallel

[rime]

An Avaya IP DECT terminal ( XMOBILE) is bridged onto the station of a call center agent. The DECT rings in parallel with the IP phone when I call the extension, it also rings in parallel when I ring the agent login, but it does not when I call the skill's extension or when a call arrives via a VDN/vector to the skill. How can I configure that the DECT device rings in parallel for ACD calls? Thanks in advance!