Hello, not sure what a VoIP is. However, your initial question is a good one. If there was sufficient detail in the oral exchanges -- such that the material terms of the agreement were before your client -- and he agreed to that, the subsequent non-conforming written document cannot be used to vary those terms. A caveat -- if the agreement is for a period that exceeds one year, the written agreement would prevail because of the statute of frauds. I'm talking now about the oral contract, not the written. If the oral agreement was for a period specifically exceeding one year, the statute applies. If not, it doesn't and the client's oral agreement prevails.