Avayaaacc,
Pay particular attention to what randycarrol says about phones registering to PROCR. It's important. PROCR is the registration point for media gateways, H.323 phones and some other services. If you're using SIP phones, some of this doesn't apply. When your h.323 IP phone register's to PROCR, Communication manager has to make a decision about where to map the phone in it's logical map of the different network regions of the system. You may have only one network region or up to a couple of thousand. It does this by comparing the IP address of the phone to the IP-Network Map. Check yours by "display ip-network-map".
If yours is empty, your phone will be mapped to the same network region as PROCR itself. Avaya best practices advise that PROCR should have it's own network region with no media resources. To adhere to this, you must populate the network map. What happens next? Your phone will be assigned the new network region and from that network region receive it's list of backup media servers. CM will use information from the network region to determine things like which codec to apply to your calls and, as kyle555 says, which ARS/AAR rules to search when you place calls. It also uses the network region to make DSP assignments when placing calls. When you pick up the handset, CM tries to assign a DSP from your own home network region, or if there is none, it will check adjacent network regions based on the inter-network region connections programmed on your phones own network region.
For instance, if your local office gateway runs out of DSPs, you probably have a direct media connection to your data center which may have a free DSP with which to provide dial tone.
Whether you place your office phone in the data center network region or the office network region depends in large part on which DSPs you want to use. If you have local PRIs which you prefer to use for your outbound calls, your office phone should probably be in the office network region. What should you do if you get rid of the PRI and centralize your trunks? That may or may not have a different answer.
Hope this helps and I hope you have a diagram of your network regions. I had to diagram my own and it's not easy when you have 150+ locations.