I'm using the latest 5.4.4 release. This is a plain old SIP extension, not a BLF key or anything else.
The phone configuration looks like this:
attendant.resourceList.2.address="7060"
attendant.resourceList.2.label="7060"
feature.dect.enabled="1" msg.mwi.1.subscribe="7040" msg.mwi.1.callBackMode="contact" msg.mwi.1.callBack="*97" reg.1.displayName="7040" reg.1.address="7040" reg.1.label="7040" reg.1.auth.userId="7040" reg.1.auth.password="xxxx" reg.1.lineKeys="1" reg.1.terminationType="VVX-DECT" VVXD60.Handset.1.outGoingLineIndex="1" VVXD60.Handset.1.line.1="1"