[Home](./index.md) > [sip.js](./sip.js.md) > [Transport](./sip.js.transport.md) > [onDisconnect](./sip.js.transport.ondisconnect.md) ## Transport.onDisconnect property Callback on state transition from "Connected". Signature: ```typescript onDisconnect: ((error?: Error) => void) | undefined; ``` ## Remarks When the `UserAgent` is constructed, this property is set. ```txt - The `state` MUST NOT "Connected" when called. - If prior `state` is "Connecting" or "Connected", `error` MUST be defined. - If prior `state` is "Disconnecting", `error` MUST NOT be undefined. ``` If the transition from "Connected" occurs because the transport user requested it by calling `disconnect`, then `error` will be undefined. Otherwise `error` will be defined to provide an indication that the transport initiated the transition from "Connected" - for example, perhaps network connectivity was lost.