In my opinion. Field workers are workers whose employment is ALWAYS away from the employer's premises or office and as a rule they are not entitled for OT pay, simply because they may or may not observe the 8 hours work. SO pwedeng malugi ang companya if palagi silang bayaran ng OT.
However, This is contrary to your employment because your deployment to the province is NOT ON REGULAR basis but on occasion lang, as what youve said, you are assigned "sometimes" and not all the time away from the office, year in and year out.
For me, you are not considered as a field employee. Thus, even you are deployed in the province and rendering OT work, STILL you are entitled for OT pay. Furthermore, basic as a rule in labor law that in case of doubt as to your employement classification, such doubt should be resolved in favor of the working class. Which means, it is favorable for you NOT to be considered as a field worker to avail the benefit of the OT pay, than to be considered as a field worker.