have a second string to (one's) bow
To have another way to accomplish something. At least you have a second string to your bow, since you can always contact your aunt if the recruiter from HR doesn't call you back.
have a second string to your bow
have an alternative resource that you can make use of if the first one fails. British This is a metaphor from archery; related expressions include have several strings to your bow and add another string to your bow. Second string can also be used on its own to mean simply ‘an alternative resource or course of action’.