Agent communication: email, voice and phone: 7 companies ranked

Companies selling inboxes, phone numbers and voice for AI agents, in SalesBook agentic-sector rank order. Every company has a profile with dated public sources and a live check of its agent interfaces (MCP, llms.txt). Ranks use the published SalesBook method; tell us who is missing.

• Vapi#5Developer and enterprise platform for building, testing and deploying voice AI agents for phone and web calls.
  MCP connector: nollms.txt: yes
• Synthflow#9Enterprise AI voice agent platform with its own telephony, used by contact centers, BPOs and enterprises to automate phone calls.
  MCP connector: yesllms.txt: no
• Retell AI#20Voice AI agent platform for inbound and outbound phone calls, with chat and SMS, sold to healthcare, financial services and other enterprises.
  MCP connector: yesllms.txt: yes
• LiveKit#21Open source framework and cloud platform for building and running voice, video and physical AI agents, incl. telephony and inference.
  MCP connector: nollms.txt: yes
• AgentMail#41Email inbox API for AI agents: create inboxes, send, receive, thread and search email, with free and paid plans.
  MCP connector: yesllms.txt: yes
• Cartesia#65Real-time text-to-speech (Sonic), speech-to-text (Ink) models and a managed platform for enterprise voice agents.
  MCP connector: yesllms.txt: no
• Inkbox#102Identity and communication layer for AI agents: email inboxes, phone numbers, iMessage, webhooks tunnel and credential vault.
  MCP connector: yesllms.txt: yes