Getting toll free phone lines!

I work for an NGO in Cambodia trying to start a mhealth project providing new mothers and pregnant women with health info accessed by their mobile phone. By using an IVR system, mothers dial a (hopefully toll free) phone line, and use their key pad to access the relevant pre-recorded voice messages according to their month of pregnancy or age of child.
My question is: does anyone have experience in getting phone companies to provide a free (or subsidised) phone line (especially in Cambodia)? Worryingly, I can't find any similar projects with a free or low cost phone line.
Thanks, Emma