Work from the Computer?

i am currently looking to make sales calls to the united states from here in asia. do any of you do social media marketing from outside the united states? my question is how do you make calls from here in asia to the united states? do any of you recommend a VOIP phone calling software if so what should i use? the biggest thing i need to know is how can i make calls from here in asia to the united states.. preferably unlimited calls.