CallMCP — Phone Call MCP Server

Attach Phone Number

attach_number

DestructiveIdempotent

Assign a phone number already in the KaiCalls registry pool to a business (and optionally route it directly to an agent). Does NOT purchase a new number; search_available_numbers and buy_number handle purchases.

Tool call template

Replace each placeholder with a value matching the input schema. Only required parameters are included; add optional parameters as needed.

{
  "id": 1,
  "jsonrpc": "2.0",
  "method": "tools/call",
  "params": {
    "arguments": {
      "phone_number": "<phone_number>"
    },
    "name": "attach_number"
  }
}

Input Schema

Name	Required	Description
`agent_id`	No	Agent ID to route this number to directly (optional).
`business_id`	No	Business ID (optional only when the token can access exactly one business).
`phone_number`	Yes	E.164 phone number already in the registry.

Output Schema

Name	Required	Description
`error`	No	Present when success is false
`number`	No
`success`	Yes	Whether the tool completed successfully
`business_id`	No

Schema Changelog

Changes observed during successful MCP inspections.

First observedNov 12, 2026

TDQS

A4.2/5.0

Behavior3/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

The annotations already declare destructiveHint=true, idempotentHint=true and openWorldHint=true, so safety and repeat-call behavior are largely covered. The description adds one genuinely useful fact — that no purchase occurs, implying no charge — but never explains the destructive side (e.g., whether assigning displaces an existing business or overwrites agent routing). With rich annotations, this partial add-on merits a 3.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

Two sentences, zero filler, with the core action front-loaded and the disambiguation clause immediately following. Every clause carries information the agent needs.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness4/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

An output schema exists and annotations cover mutability, so return values need no explanation here, and the description covers purpose, cost semantics, and alternatives. It falls short only on mutation consequences — what happens to a previously attached owner or agent route — which matters for a tool flagged destructive.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 100%, so all three parameters, including the E.164 constraint and the single-business token caveat, are already documented in the schema. The description only restates that an agent can be routed 'directly' and adds no format, validation, or precedence guidance beyond that, so the baseline 3 applies.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource — 'assign a phone number already in the KaiCalls registry pool to a business' — plus optional routing to an agent. The scope qualifier 'already in the registry pool' cleanly separates it from purchase-oriented siblings without ambiguity.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines5/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

Explicitly states what the tool is NOT for ('Does NOT purchase a new number') and names the alternatives that handle that path (search_available_numbers, buy_number). The registry-pool wording also implicitly excludes attach_own_twilio_number, giving the agent enough routing information without opening other schemas.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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