Corvisa is a well-known B2B telephony service provider that specializes in cloud PBX and a call center systems. Corvisa pricing is available only upon request. If you are looking for a free Corvisa alternative that functions as virtual PBX, a unified communication platform and call center, Bitrix24 is a great option. First, Bitrix24 is free for 12 agents. Second, it comes with a CRM. Incoming calls are free and outbound calls are dirt cheap (2 cents per minute without connection or first minute fees). You can also rent local, toll-free and international phone numbers in 60+ countries for only $6/mo. Finally, Bitrix24 is available both in cloud and on-premise (with open source code access).
* Please note that the information may have changed since the publication. For current prices and features please visit Bitrix24 pricing page.