# Telephony Simulator

## Telephony Simulator

The Telephony Simulator is a tool that enables you to test **Telephony SDK** integration without the dependency of provisioning real phone numbers and sending text messages across the wire.

## The "ZZ" Country Code

The "ZZ" country code is included in the [list of supported countries](/guides/telephony/phone-numbers.md#get-supported-countries) for phone provisioning in projects that are in sandbox mode. Provisioning phone numbers in this region does not incur any number provisioning costs, as the resulting numbers are "simulated" and cannot communicate with other phone numbers.

Outgoing SMS and MMS messages from simulated numbers will by default be marked as successfully delivered. However, this behavior can be changed by sending a message to a test number as described below.

## Test Numbers

There are a few phone numbers that have special behavior in the simulated telephony region. These test numbers can be used to exercise specific code paths that may be difficult to test otherwise.

**Note:** The special behavior for these test numbers only occurs when using a phone number provisioned in the simulated telephony region.

| Test Number  | Behavior                                                                                                                                   |
| ------------ | ------------------------------------------------------------------------------------------------------------------------------------------ |
| +11111111111 | Attempting to provision this phone number with the simulated telephony region will always fail.                                            |
| +12222222222 | Messages sent to this test number will end up in the ["Undelivered" state](/guides/telephony/text-messaging.md#phone-message).             |
| +13333333333 | Messages sent to this test number will end up in the ["Failed" state](/guides/telephony/text-messaging.md#phone-message).                  |
| +15555555555 | When an SMS or MMS message is sent to this test number, a SMS reply will be automatically generated in response.                           |
| +17777777777 | When an SMS or MMS message is sent to this test number, an MMS reply with an image attachment will be automatically generated in response. |


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