Help setting up hookflash trunk?

I think this is exactly what I want:

If you want the user to dial 900, hear dialtone from the Valcom and then dial the required code, set your Outbound Route with match pattern 900 and set the dial plan in the Grandstream to replace 900 with an empty string. Some FXO devices won’t allow an empty string, so try replacing 900 with ‘a’ (without the quotes).

Grandstream info:|Product Model: | HT-503 V2.0A|
|—|---|
|Software Version: | Program – 1.0.16.2|

Not sure how to alter the dial plan in the grandstream though