[asterisk-users] How to limit extension to allow only internal calls

[DEVEL STAROSTA]

Hi all,

I'm playing with asterisk and I try to make system for my sister hotel 
with aprox. 40 extensions (30 rooms, 1 reception, 2 office, 4 services 
phones on each floor). I.ve installed latest version of trixbox (for 
GUI) but I made manual modifications.
This is actual server:
- n.1 Pci card with 2 x FXO + 2 x FXS
- n. 2 Trunk to outside (my local Voip provider + 1 PSTN)
I've buyed also 50 telephones from ATCOM (IP530), make all 
configurations and now seems all work fine, I can make calls (outside, 
inside) make transfer, ecc.

My problem:
I have 4 services phones (each per floor) which must be able to make 
only internal calls (not external - no PSTN nor VOiP provider), I've 
just look about solution in Google, mailing list, I can't find solution, 
please can someone give me an idea ?
NB: sorry for my english ... ;)

Alberto