You can purchase a simple toy voice distorter, which will make your voice sound unnatural. It makes the voice robotic or a really high pitched “chipmunk” voice.
You can also go for an advanced version of the system - the digital voice changer. It allows the users to change their voices to any gender and any age. If you are a female and want to sound like a male, then you only need to set the mode to male and define the age limit. Now, whatever you speak through the voice changer will be converted into male voice.
If you want your voice to appear natural to the other person, then you should try not to alter the settings too dramatically.
You can also consume some helium before you talk on the phone. Helium will make your voice really high-pitched and the other person will not be able to identify you. Here, you should keep helium besides you while you speak on the phone, as the effect lasts not more than one minute. Therefore, you will need to consume it more when your voice starts sounding natural.
You can also cover the receiver of the phone with a towel and the muffled voice may become unrecognisable to someone, who does not speak to you regularly on the phone.
Changing the pitch of your voice is perhaps the easiest of all tricks. You can just raise your head and speak from the upper portion of your neck. It will make your voice really high pitched. If you want your voice to be pitched low, then you can just lower your chin to your chest and speak.