You can purchase Telephony & SIP connectors for Bitrix24 On-Premise versions here.
Please note that the active subscription is required for your Bitrix24 On-Premise if you'd like to use Bitrix24 Telephony.
To continue to use the product, receive updates & support afterward, annual subscription cost is set at 50% of the price for the On-Premise Bitrix24 edition you purchased. Read more in the article - Switching Bitrix24 On-Premise license to a subscription model.