Make sure the patron understands the answer. It helps to begin by making sure the patron is ready to take down the answer. You can offer to spell difficult words or names and check to make sure the patron heard. In giving information, always identify the source of the information first, before giving it. Both the title and date of the source are important so the patron can evaluate the currency and accuracy of the information.
Some libraries create a special policy for answering email reference question.
Let the patron know who you are in case they want to contact you. Get the patron's name and phone number, and repeat the spelling and number. Likewise, with an email account. It is easy to misunderstand an email address given over the phone. Having a phone number to fall back on insures delivery of the answer to your customer.