Weird cell phone question

I just got a Samsung smartphone (for my stay in Phnom Penh this summer) with a Movitel sim card. I'm trying to figure out which is the cheapest way to call the US: through the cell phone itself (using the 177 prefix seems to make it pretty cheap) or by activating the phone as a hotspot and using skype (thereby using up my data on my phone card). The phone doesn't give me any info on my balance until it is under $1, and my Cambodian friends didn't have much info on how much data usage costs (although they say that the 177 prefix should make intl. calls around .06-.10 USD per minute). Anybody have any advice?
Thanks so much!