Workflow Modify Lead Associated With Call

I have a custom field in leads 'Last Called' for date/time when lead was called.

I setup a work flow for the Calls module.

One condition in that the status = 'held'

Repeated runs, only on save, all records

Action is Modify Record, Leads:Leads, Last Call field = field End Date

I can see in the process audit of the workflow it is being triggered, but it isn't modifying the Last Called field in Leads with the End Date field of the Call associated with that lead.

Is this because the record type is Lead : Lead and not Call : Lead ? But that record type isn't available even though calls can be associated with leads!