Try InvokeCommandAction a command instead of using CallMethodAction:
<i:Interaction.Triggers>
<i:EventTrigger EventName="TextChanged">
<i:InvokeCommandAction Command="{Binding TextChangedCommand}"
CommandParameter="{Binding ElementName=filterBox, Path=Text}"/>
</i:EventTrigger>
</i:Interaction.Triggers>
Hope it helps
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