Do I have to pay US tax when I am not from the US?

I own a small telemarketing business outside the US and makes some sales calls to US clients. My business is offering promoitonal items so shipping is not a problem because I purchase items from US suppliers and ship it out to US customers if I have sales. I invoice my US customers by using a US virtual office and then we received checks payable to our company name addressed to our US virtual address. Am I required to pay tax? I am not a US citizen and I do not live in the US and my business is not located in the US. My business is an online business. It is only our virtual office in the US and invoice customers by using our virtual address. Please help. Thanks.