3CX - Hosted PBX with CRM integration and mobile apps
Linphone - Linphone is an internet phone or Voice Over IP phone (VoIP).
Microsoft Teams - Microsoft Teams provides the enterprise-level security, compliance and management features you expect from Office 365, including broad support for compliance standards, and eDiscovery and legal hold for channels, chats, and files.
Zoiper - Cross-platform VoIP softphone dialer for SIP and IAX systems
Amazon Connect - A self-service, cloud-based call center 📞
Sipdroid - Free and open-source SIP/VoIP Client