They were originally my Grey and Snowy, actually. And, yes there is White in the bloodline. Together the Grey and Snowy have produced 5 pure bred white Calls.
Then there is a chance of pure... but I would still err on the side of mixed due to having a Pekin drake in the same pen... but that's my personal opinion...

I'm super strict about my Calls, no drakes of any other breeds and the only 2 ducks not Calls have easily recognizable eggs that I will not hatch...