For testing 911 some carriers support automated testing by dialing 933. I don't recall if Telnyx supports it, but give it a try. When you call it will read to you the address associated with the DID you are calling from - if one is configured.
If you cannot test with 933, go ahead and call 911. Just tell them "This is not an emergency, we are testing a new phone system and you need to verify that 911 is setup correctly."
Newer versions of 3CX include a special Emergency Numbers section in Settings, General. Go there and add 911 and 933, then set the route to your Telnyx trunk. The advantage of setting up this way is these numbers can be dialed at any time by any extension, even if your license limit has been reached or the extension is not allowed to make outbound calls.
For complicated situations where you have some extensions that are at a different site, but you want the outbound caller ID to always be the "main" company number, you can setup a new trunk that forces a particular DID to be used when 911 is dialed. You can do this for most trunks by modifying the 3CX Outbound Parameters to force the Trunk DID instead of the Global or Extension outbound DID, but with Telnyx you can setup a connection with Outbound settings that uses Caller ID Override for Emergency calls. It works very well - just setup a separate trunk for the connection and only use it for Emergency Calls. Feel free to PM me if you need a walk through of this with Telnyx.
