You can subscribe via SMS or USSD code:
Subscribe by SMS
SMS 3001 to 2525.
SMSs sent from a prepaid subscriber to 2525 are free.
Each SMS from a postpaid subscriber to 2525 costs 0.01 AZN.
Subscribe by USSD Code
Dial *100*3001#YES on your phone.
Please SMS balans to 2525 or dial *100#YES on your device to access balance information.
Each SMS to 2525 costs 0.01 AZN.
You can see the full list of countries by the link.