Cover the phone with a towel or other piece of thick fabric. Practice with a friend who knows you well to see if the disguise is working. Raise or lower your pitch also if the fabric doesn't change your voice enough.
Adopt a foreign accent or other dialect. Classes are available if you're really serious. Changing your voice could lead to a career in voice-overs or impersonation if you're any good at it.
Buy a digital voice changer. The price can be anywhere from $15 to more than $50. This device allows you to adjust your voice higher or lower. Voice changers typically don't emit a voice that's highly believable, so save this option for the people you want to scare, intimidate or stalk.