you just need to use a different variable to store the modified
destination number.
here are a couple of examples from my blog:

http://txlab.wordpress.com/2014/10/21/using-voxbeam-for-outbound-calls-with-freeswitch/
http://txlab.wordpress.com/2013/08/03/free-us-number-and-caller-id-manipulation/

On Fri, Dec 5, 2014 at 11:22 AM, Paul Klenk <markus.klenk@googlemail.com> wrote:
> Hi,
>
> I'd like to have the country code "+49" removed from
> destination_number and changed to "0"
>
> I tried:
>     <extension name="international" continue="true">
>       <condition field="destination_number" expression="^\+49(.*)">
>         <action inline="true" application="set" data="destination_number=0$1"/>
>       </condition>
>     </extension>
>
> but after correctly executing this extension-statement it reverts to
> the original version of destination_number
>
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