In a FreePBX forum post, Alex Edwards wanted to accomplish the following:

"I'd like to warn my FreePBX users when they're dialling an expensive international fixed-line, or worse, international mobile. (This is from the UK).

"I've put a long list of international codes as a route, with a route password - which is a great start.

"However, ideally I'd like to play an explanatory message too - possibly a Misc Destination / Application / Announcement? ....."

In other words, what he wanted to do was execute a bit of custom code (a dial plan fragment) prior to sending the call to a trunk. The way to do this is to use a CUSTOM trunk. Here's how Alex did it (slightly revised for clarity):

First, you need to add your custom dial plan fragment to extensions_custom.conf (if you have the Dialplan Injection module installed, you could also put this code fragment there, but for the moment we'll assume you're using extensions_custom.conf). Alex added the following to the top of that file (slightly modified for clarity):

Note that the final line gives the actual destination of the call, and will have to be tweaked for your own particular situation. Also, the Wait statements are probably optional, or could be made longer than one second if desired. The recording specified in the fourth line (International-Mobile-Warning) is a custom recording that tells the caller that the call will cost around £1 a minute, so look for a cheaper alternative! You will need to create the recording you want to use, if none of the standard Asterisk recordings will work in your situation.

Second, create a new CUSTOM trunk. For the Custom Dial String, use the following:
Local/$OUTNUM$@mobile-warn-custom

If you used something other than "mobile-warn-custom" to identify your custom dial plan then be sure to use that here as well. While you are creating your CUSTOM trunk, mouse over the words "Custom Dial String" and it will show you some suggested formats that might give you an idea of how the "Dial" argument in the final line of your custom dial plan should read (in case you are having trouble figuring that out).

Finally, create your route, and make sure it has higher priority than all the other routes that might otherwise handle calls to the same numbers. Don't forget to click the orange bar when finished.

You may want to check the forum thread to see if any additional comments have been posted that may clarify this procedure. The thing we want to illustrate here is that when you need to add a custom dial plan fragment prior to sending a call to an actual trunk, the way to do it is to use a CUSTOM trunk, send that to your custom dial plan, and then make the last line of your custom dial plan send the call to an actual trunk or other destination.

For those who may be trying to duplicate what Alex did, here is the list of numbers that Alex put into the route definition (to play a warning for high cost mobile calls). Note that this may or may not include all caller-pays mobile prefixes, and that the '9|xxxx.' variants at the bottom of the list are only required if you use 9 for an outside line: