I could find nothing in the SK PTO rulebook to indicate that WP must be fired before any real bullets during DFF. Here is the WP section of rule 6.2 Special Ammo:
WP (White Phosphorous): WP may only be
fired on the ATT and at the start of the PFPh
or the DFPh or as Defensive First Fire. A
unit hit with WP must take a NMC, with the
TEM subtracted from the NMC DR; Mortar
Air Bursts from WP add a +1 to the DR. A
WP CH adds the positive TEM to the DR.
Treat WP as Ordnance Smoke, except that
full WP is a +2 counter and Dispersed WP is
a +1 counter.
The only mention of smoke in rule 3.3.3 Defensive Fire is about extra MF spend by infantry to toss smoke a grenade being eligible for a shot. There is no mention of WP in that rule section. I thus conclude that WP can be fired as DFF by the player turn defender during the opponent's MPh at any point without limitations based on other units' firing. The WP rule does say to treat WP as Ordnance Smoke except counter strength. I could find nothing about Smoke in the DFF, other than it is not allowed. I don't think we should should make up restrictions about it to limit WP usage during that phase.
I will add that for AFV Vehicular Smoke Dispenser usage during its MPh, there IS a restriction stating that each vehicle may NOT use a Smoke Dispenser if that vehicle has fired any weapon (including BFF or DFF) [rule 7.5 Vehicular Smoke Dispensers]. It does seems appropriate to apply THAT to all Ordnance Smoke attempts during DFF, meaning OTHER things can fire "real bullets" - and un-fired Ordnance can still shoot WP without restriction. But if a large MTR has fired an HE shell, it may no longer try for WP - regardless of whether or not other units have fired. But even this interpretation is not truly cut and dried.