Do not cofuse WILCO with ROGER.
As 'sevenstrokeroll' says, WILCO = Will Comply
However, ROGER simply means I HEAR WHAT YOU SAY* - a totally different concept!!!
* technically 'I have received your transmission satisfactorily'
Who would respond WILCO to something they did not understand?
I think if you look in ICAO Doc 9432 you'll find that WILCO means "I have understood the message and will comply" (or words to that effect)