There are ways to disable Google Voice or unsubscribe completely. However, one thing to keep in mind is that if you use Google Voice with an existing Sprint account or service, or if you paid Sprint to transfer your number, you won't be able to delete it. Furthermore, deleting your Google Voice number will not erase your account's data so that messages will remain in your inbox. To learn how to delete Google voice number on iPhone, you need to go into the settings and select "Phone." Once you're in the Voicemail menu, choose "Voicemail" and then tap on the arrow next to the number you want to delete. Then, click "Delete," and then confirm your choice.
See also - how to find Google Voice number