You need to reference the subform through the mainform because the
subform is not "open" as per say in the [forms] collection

Use the condition

[ID]=[Forms]![ frmSwitchboard]![subfrmswitchboard]![ID]

If you open this subform as a normal form as well then tell me and you
need to do something more complicated as this condition will not work
in that case.

Regards
Kelvan