hi4hi muffler doesn't uses the internet connection and hence it neither collects nor transmits any information outside of your device. The only information that goes out of your device is the SMS to the caller telling when your phone will come out of silent mode and that is a feature.
I will talk a bit about the permissions that hi4hi muffler uses and why.
The following permission are used to detect the caller and send sms. Sending sms can be disabled from the settings, in which case the app will ignore missed calls.
These are needed to know the number of the caller and send SMS when your phone is on silent mode.
The permissions below are all used for generating notifications. Notifications can be disabled from the settings.
This is to check if received SMS is from hi4hi muffler. It can be avoided but the alternative will cost battery life.
This is required to show the name of the contact instead of the number.
READ_SMS & WRITE_SMS
These are needed to delete the auto-sms from the hi4hi muffler app to keep your messaging history non polluted.