You can purchase Telephony & SIP connectors for Bitrix24 On Premise versions here.
Please note that the active maintenance subscription is required for your Bitrix24 on premise if you'd like to purchase Bitrix24 Telephony.
All updates and priority support are 100% free your first year. If you’d like to receive those afterward, annual subscription cost is set at 25% of the price for the on premise Bitrix24 edition you purchased.