Minimum rendered time period to be eligible for overtime pay

just wanted to inquire on what is the minimum time outside of the 8 work hour that will make me entitled to overtime pay. in the company im employed with, a call-center, our working hours are from 8am to 5pm from m-f. however every so often we are asked to stay 30 mins longer on the phones to offset the time we used for our short breaks in order to comply with the supposed client's directive that we need to complete 8hrs a day during weekdays.

are we then entitled to receive ot pay even if its only 30 mins? thing is, its stipulated in the employment contract which i signed that i have to render 44 hrs a week. and though we sometimes do saturday shifts, half-day at that, it isnt on regular basis. can my employer use this as a means to counter such ot pay requests? looking forward to your advice please. thanks!

the law is silent, but a strict interpretation would mean 1 minute above 8 hours should already be considered overtime