I am explaining how do we change doorbell tone(GND-BELL) and ringtone tone in this topic.
Scenario: There are 2 Akuvox doorphone in the project. Also You have 1 Qty Akuvox indoor monitor(C313). You want to sound different ringtone tone when anything came to call from anything doorphone.
Firstly You have to add as contact to indoor monitor the doorphone.You can select the ringtone you want according to the outdoor unit you want to add.
First For Doorphone unit:

Second For Doorphone Unit:

If you want to change doorbeel tone you can make on indoormonitor screen. You need to just change Ring Tones section.After choose PleaseYou shouldn't forget to press button.
