How Limit phones on dialing out

Hi

I have asterisk setup with 2 sip phones ext 104 and 105 and a analogue line.

Its setup so both phones can dial eachother and you must dial 9 for outside line.

I now wish to limit ext 105 on outgoing dial so he can only dial the emergancy services on 9,999 and im not sure on how to do this.

I have tried having a second context like office and putting ext 105 into it along with include => outgoing-Emergancy-only for dialing 999 but then the phones cannot dial eachother and if I add include => office in the new context then he is able to dial out which I dont want.

Any pointers on how to sort this out.

Thanks

My Config

sip.conf
[104]
type=friend
username=104
secret=1234
host=dynamic
context=office

[105]
type=friend
username=105
secret=1234
host=dynamic
context=office

extensions.conf

[office]
include => outgoing-dial
exten => 104,1,Dial(SIP/104,20)
exten => 105,1,Dial(SIP/105,20)

[outgoing-dial]
exten => _9X!,n,Dial,Zap/g5/141${EXTEN:1}