You can execute the following commands using USSD codes on your SIM or eSIM card.
Simply enter the code on your phone keypad and click "Call" to send it.
USSD codes:
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Check balance: *146*099#
Activate voicemail: *146*091#
Deactivate voicemail: *146*090#
Check number of new messages in voicemail: *146*094#
Listen to voicemail: *146*095#
Current data allowance balance: *146*942#