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Resume a held call

unhold_call
DestructiveIdempotent

Take a live call leg off hold by transferring it to an approved VobizXML resume URL that controls the next step. Requires confirmation; use only after hold_call.

Instructions

Take a leg of a LIVE call off emulated hold by transferring it to resume_url (VobizXML on an approved host, for example a hosted flow from generate_answer_xml or the url from gather_dtmf). Vobiz cannot return to the point the call was at before hold_call, so the resume XML decides what happens next (speak, dial the other party again, a menu). To end the call instead use end_call. Only meaningful after hold_call; a call that is not live fails with call_not_active. Requires confirm: true.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
legsNoLeg to resume: the same value given to hold_call (default aleg).
confirmYesMust be true, set only after the user explicitly asked to take this call off hold. If your client supports MCP elicitation, the server asks the user to approve directly and this flag is not relied on; otherwise it is your attestation that the user approved.
call_uuidYesUUID of the LIVE call to act on: the request_uuid returned by make_call, or a UUID from list_live_calls.
resume_urlYeshttps URL on an approved host returning the VobizXML that controls the leg after hold.

Output Schema

TableJSON Schema
NameRequiredDescriptionDefault
okNoTrue on success. On a failure the result is isError with ok: false plus `error` (kind), `message`, `retryable` and sometimes `http_status`.
legsNoThe leg(s) the action was applied to (aleg when none was given).
on_holdNoFalse once Vobiz accepted the transfer to resume_url.
emulatedNoAlways true: hold and resume are transfers between hosted flows, not a native Vobiz hold.
call_uuidNoThe call UUID the action was applied to, echoed from the request.
http_statusNoHTTP status Vobiz answered with: 202 when an action was accepted, 204 when one was stopped.

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed5 schema fields changedv1.4.2
    • addedInput schema / properties / call_uuid / description
      Added value: +"UUID of the LIVE call to act on: the request_uuid returned by make_call, or a UUID from list_live_calls."
    • changedInput schema / properties / confirm / description
      Previous value: -"Must be true, set only after the user explicitly asked to take this call off hold."New value: +"Must be true, set only after the user explicitly asked to take this call off hold. If your client supports MCP elicitation, the server asks the user to approve directly and this flag is not relied on; otherwise it is your attestation that the user approved."
    • changedInput schema / properties / legs / description
      Previous value: -"Which call leg: aleg (default), bleg or both."New value: +"Leg to resume: the same value given to hold_call (default aleg)."
    • changedInput schema / properties / resume_url / description
      Previous value: -"XML URL that should control the call after hold (approved host)."New value: +"https URL on an approved host returning the VobizXML that controls the leg after hold."
    • changedOutput schema / (root)
      Previous value: -nullNew value: +{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "additionalProperties": {},
+  "properties": {
+    "call_uuid": {
+      "description": "The call UUID the action was applied to, echoed from the request.",
+      "type": "string"
+    },
+    "emulated": {
+      "const": true,
+      "description": "Always true: hold and resume are transfers between hosted flows, not a native Vobiz hold.",
+      "type": "boolean"
+    },
+    "http_status": {
+      "description": "HTTP status Vobiz answered with: 202 when an action was accepted, 204 when one was stopped.",
+      "maximum": 9007199254740991,
+      "minimum": -9007199254740991,
+      "type": "integer"
+    },
+    "legs": {
+      "description": "The leg(s) the action was applied to (aleg when none was given).",
+      "enum": [
+        "aleg",
+        "bleg",
+        "both"
+      ],
+      "type": "string"
+    },
+    "ok": {
+      "description": "True on success. On a failure the result is isError with ok: false plus `error` (kind), `message`, `retryable` and sometimes `http_status`.",
+      "type": "boolean"
+    },
+    "on_hold": {
+      "const": false,
+      "description": "False once Vobiz accepted the transfer to resume_url.",
+      "type": "boolean"
+    }
+  },
+  "type": "object"
+}
  2. First observedv1.1.1

TDQS

A4.9/5.0
Behavior5/5

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

Annotations already declare destructiveHint=true, idempotentHint=true and openWorldHint=true, but the description adds context the annotations cannot: the irreversibility of hold (Vobiz cannot return to the pre-hold point), the fact that resume_url XML dictates subsequent behavior, the approved-host constraint on the URL, and the confirm:true approval requirement.

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?

Three sentences, front-loaded with the action and its mechanism, then the alternative, then the precondition. Dense with no filler and no redundancy.

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

Completeness5/5

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

An output schema exists, so return values need no explanation. Preconditions, error behavior, URL constraints, irreversibility, and the sibling alternative are all covered, which is everything an agent needs to invoke this correctly.

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

Parameters4/5

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

Schema description coverage is 100%, so the baseline is 3. The description earns a bump by explaining the semantics behind resume_url (VobizXML on an approved host) and pointing to generate_answer_xml and gather_dtmf as example sources, which the schema does not say.

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 (take off hold) plus resource (a leg of a LIVE call) and the mechanism (transfer to resume_url). It also distinguishes itself from siblings by naming end_call and referencing hold_call as the required predecessor.

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?

Gives explicit when (only after hold_call on a live call), when-not (to end the call use end_call instead), and the failure condition (call that is not live fails with call_not_active). Nothing material is left to inference.

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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