I actually think 3 PRI's would be more than adequate. 1 to 5 trunking is a tad low but consider what you are doing here as well. It's only a bit under 25% and the PRI helps alot in this regard. If this is for housing you maty really want to consider using a PBX with analog ports that provide caller ID. As far as the 2 buildings I assume they are close enough to be cabled together with feed cable? Without knowing the specifics I wouldn't try and second guese you. The 4th or 5th PRI if you get them you may consider getting a dedicated one from a long distance provider to give you premo rates on long distance and 800 numbers.

Coral Tech
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