This excerpt taken from the VZ 10-K filed Feb 28, 2008.
Concentrations of Credit Risk
Financial instruments that subject us to concentrations of credit risk consist primarily of temporary cash investments, short-term and long-term investments, trade receivables, certain notes receivable, including lease receivables, and derivative contracts. Our policy is to deposit our temporary cash investments with major financial institutions. Counterparties to our derivative contracts are also major financial institutions. The financial institutions have all been accorded high ratings by primary rating agencies. We limit the dollar amount of contracts entered into with any one financial institution and monitor our counterparties credit ratings. We generally do not give or receive collateral on swap agreements due to our credit rating and those of our counterparties. While we may be exposed to credit losses due to the nonperformance of our counterparties, we consider the risk remote and do not expect the settlement of these transactions to have a material effect on our results of operations or financial condition.