# Twilio compatibility OneML is compatible with Twilio's TwiML, and PressOne's REST surface mirrors Twilio's `2010-04-01` Calls API, so most existing TwiML documents and Twilio integrations run against PressOne unmodified. This page is the exception to the rest of this documentation: it names Twilio and TwiML directly, and exists specifically to help developers migrating an existing Twilio integration understand where PressOne's behavior differs. ## Supported verbs Each OneML verb below links to its full reference page. | OneML verb | Twilio verb(s) | Notes | |------------|-----------------|-------| | [Say](/voice/oneml/say) | `` | Voice and language defaults differ. See [behavioral divergences](#behavioral-divergences). | | [Play](/voice/oneml/play) | `` | | | [Gather](/voice/oneml/gather) | `` | | | [Record](/voice/oneml/record) | `` | | | [Dial](/voice/oneml/dial) | `` | Only a bare number or `` children are dialed. See [not supported](#not-supported). | | [Redirect](/voice/oneml/redirect) | `` | | | [Pause](/voice/oneml/pause) | `` | | | [Hangup](/voice/oneml/hangup) | `` | | | [Reject](/voice/oneml/reject) | `` | | | [Stream](/voice/oneml/stream) | `` | Bidirectional, blocking media session. | | [Stream](/voice/oneml/stream) | `` / `` | One-way, non-blocking media session. | | [Disconnect](/voice/oneml/disconnect) | No Twilio equivalent | PressOne extension. Returns control of the call to the platform dialplan without hanging up. | ## Not supported The elements and attributes below are not fully implemented. They fail in different ways depending on the case: some reject the document outright, others are silently skipped, and others are accepted but never take effect. Knowing which applies to a given document matters when migrating: a hard error is visible immediately, while a silent skip is not. ### Rejected with a validation error | Verb | What happens | |------|---------------| | `` | Parses as valid XML, but fails semantic validation with the error ` is not supported yet` when PressOne executes the document: the call ends as failed. This applies wherever a ``-containing document reaches the engine: an initial webhook response, a `` target, or an inline `Twiml` document. For inline `Twiml` supplied on create-call specifically, this check does not run synchronously against the request: the parser only confirms the document is well-formed XML at that point, so `POST .../Calls.json` still returns `201 Created`. The `` rejection only happens later, when the engine actually executes the call, and surfaces as `status: failed` on the call resource or on a status callback, not as a create-call error. | ### Accepted, skipped without effect The following elements parse and validate successfully (a document containing them passes OneML's strict parser), but the executor has no handler for them. Each falls through to a generic case that logs a warning on the server and continues with the next verb. There is no error, no `action` callback, and no signal to your webhook or to the caller that anything was skipped. | Element | Context | Notes | |---------|---------|-------| | `` | Direct child of `` | | | `` | Direct child of `` | | | `` | Direct child of `` | Its `sendDigits` attribute is validated for format, but the verb itself never executes. | | `` | Child of `` or `` | | | `` | Child of `` or `` | | | `` | Child of `` | Only `` is implemented; other `` targets fall through the same way. | | `` | Child of `` | | | `` | Child of `` | | `` and `` still require at least one recognized child (``, ``, or ``) to pass validation: an empty `` or `` is rejected before execution. Once past that check, only the `` child actually runs. ### Dial targets dropped with a warning `` executes only a bare phone number or `` children. The following nested targets parse without error, but PressOne drops them with a server-side warning when it builds the list of numbers to dial. If a `` contains only unsupported targets, PressOne skips the dial entirely and continues with the next verb, as if `` were never present. | Target | Notes | |--------|-------| | `` | | | `` | | | `` | | | `` | | | `` | | ### Dial attributes parsed but ignored The following `` attributes are recognized and validated on the parsed document, but setting them has no effect on the placed call. | Attribute | Notes | |-----------|-------| | `trim` | Recording trim behavior is not configurable through ``. | | `referUrl` | | | `recordingTrack` | | ## REST API differences PressOne's REST API mirrors Twilio's `2010-04-01` Calls resource closely enough that most existing integrations run unmodified, with the following differences. - **`ApplicationSid` is rejected.** Twilio's create-call endpoint accepts `ApplicationSid` to run a pre-configured Twilio Application instead of `Url`/`Twiml`. PressOne has no equivalent concept: supplying `ApplicationSid` always fails the request with error `21605`. Use `Url` or `Twiml` instead. - **`Timeout` defaults to 60 seconds.** The ring timeout on create-call defaults to 60 seconds before PressOne treats the call as unanswered, matching Twilio's default. - **Updating a live call requires a JSON body, not form encoding.** Every other endpoint in this API accepts `application/x-www-form-urlencoded` with PascalCase field names, matching Twilio's convention. The update-call endpoint is the one exception: it requires an `application/json` body with snake_case fields (`voice_url`, `voice_method`, `twiml`). Twilio's official helper libraries send update-call requests form-encoded, so they cannot update a live call against PressOne without modification: issue a custom HTTP request with a JSON body instead of using the helper library's built-in call-update method. - **The recordings list is not paginated.** Twilio paginates the recordings list. PressOne's equivalent endpoint returns every recording captured during the call in a single response, with no `Page`/`PageSize` parameters and no page metadata in the response body. - **Error response bodies are not uniform across operations.** Only create-call returns a Twilio-style JSON error body (`code`, `message`, `more_info`, `status`). Every other operation (list calls, get call, update call, end call, list recordings, get recording) returns a plain-text body (`Unauthorized` or `Not Found`) instead of JSON on failure. In addition, update call, end call, and list recordings collapse every failure, including an unknown `CallSid`, to a `401` response; they do not expose a separate `404` for a call that does not exist. ## Behavioral divergences - **`` dual-channel variants record a single mixed channel.** The from-answer and from-ringing start moments are honored, but `record-from-answer-dual` and `record-from-ringing-dual` do not produce separate channels per party. - **`` uses Google Cloud Text-to-Speech, not Amazon Polly.** Twilio's own TwiML reference documents Amazon Polly as the underlying engine for its Twilio-style voice names. PressOne synthesizes every `` voice, including the `man`/`woman`/`alice` aliases, through Google Cloud Text-to-Speech. - **`` is not validated against an enum.** PressOne accepts any string in `voice`, not only `man`, `woman`, and `alice`. The bare word `man` (the default) and any unrecognized bare string select a default Google male voice; `woman` and `alice`, matched exactly in lowercase, select a default Google female voice. See [Voice selection](/voice/oneml/say#voice-selection) for the full set of rules. - **`` supports a `Provider.` namespace with no Twilio equivalent.** Setting `voice` to `Google.` or `Elevenlabs.` selects a specific Google Cloud Text-to-Speech or ElevenLabs voice by name or ID. Twilio's TwiML has no equivalent mechanism for choosing a non-default voice by provider. See [Voice selection](/voice/oneml/say#voice-selection). - **`` is accepted but not applied.** Speech synthesis always uses the default voice's language, regardless of the value you set. - **`` is accepted but not applied.** PressOne validates the attribute's format but does not send the specified DTMF tones to the dialed party after the call connects. - **``'s `action` and `method` are accepted but never requested.** When a `` session ends, PressOne continues with the next verb in the document; it does not request the `action` URL. - **`` metadata omits `account_sid`.** The metadata PressOne sends to your WebSocket endpoint when a stream starts includes `account_sid` for `` but not for ``. See [Stream](/voice/oneml/stream) for the full metadata comparison. - **Status callbacks fire only for calls created through the REST API.** Inbound calls never emit status callbacks, regardless of any `StatusCallback` configured elsewhere. See [status callbacks](/voice/status-callbacks). - **`StatusCallbackEvent` does not validate its values server-side.** Any string is accepted, not only `initiated`, `ringing`, `answered`, and `completed`. Values outside that set are accepted at request time but never match an event PressOne emits, so subscribing to them has no effect. - **The signature header and webhook user agent keep their Twilio names.** PressOne signs webhook requests with a header literally named `X-Twilio-Signature`, and identifies itself with a `User-Agent` of `TwilioProxy/1.1`. Both are retained as-is so that existing Twilio SDKs and signature-verification libraries work without modification. See [verify webhook signatures](/voice/webhooks). ## Next steps - [OneML overview](/voice/oneml/overview): every OneML verb and attribute. - [Webhooks](/voice/webhooks): request format, standard parameters, and signature verification. - [Status callbacks](/voice/status-callbacks): call lifecycle events and delivery. - [API reference](/api/voice): complete REST endpoint documentation.