BLF Intercom Sort of Working

I am running Trixbox 2.4, with Asterisk 1.4.17-1. I am using Grandstream GXP-2010 phones (I know they aren't supported, but they are working for me).

I wanted to be able to just press a BLF button and have it intercom that extension. I found a snippet of code in another forum post, so I tested it on a fresh install of TB 2.4.

---------CODE SNIPPET-------
was written by Ken Williams on asterisk-users on 11-15-2006:
------
We use intercom 100% inter-office. To get FreePBX to do this with
Grandstreams by default without having to create intercom or paging
groups, just change the following line (line #58) in your
extensions.conf from:
exten => s,10,Dial(${ds}) ; dialparties
will set the priority to 10 if $ds is not null
to:
exten => s,10,SIPAddHeader(Call-Info: answer-after=0) ;dialparties will
set the priority to 10 if $ds is not null
exten => s,11,Dial(${ds})
Hope this helps someone in the future.
________________________

This works great on my fresh install. However, my production box doesn't work. In fact, the extensions.conf looks totally different.

PRODUCTION TB:
[macro-dial]
exten => s,1,GotoIf($["${MOHCLASS}" = ""]?dial)
exten => s,n,SetMusicOnHold(${MOHCLASS})
exten => s,n(dial),AGI(dialparties.agi)
exten => s,n,NoOp(Returned from dialparties with no extensions to call and DIALSTATUS: ${DIALSTATUS})

exten => s,n+2(normdial),Dial(${ds}) ; dialparties will set the priority to 10 if $ds is not null

exten => s,n,Set(DIALSTATUS=${IF($["${DIALSTATUS_CW}"!="" ]?${DIALSTATUS_CW}:${DIALSTATUS})})
-----

I have tried to make the edit, but I just get Call Failed Error Code 603 or 503. The s,n+2(normdial), is the difference between production and fresh install boxes.

What change do I need to make, in order to make this work?

Thanks!