FXO Trunks or PRI solutions for small office of 15 or less users?
What ISR and switch to use? CUE for Voicemail? how about faxing?
Quite possibly; it depends on your SP's pricing and your usage.
That's why I suggest BRI (2 channels per circuit) rather then FXO; far more reliable and with much better quality.
PRI if the users are busy enough on the phones to warrant the minimum number of channels you are allowed to order (it varies depending on provider).
BRI if you only need a few channels or PRI is too expensive.
FXO if you absolutely can't use anything else - try seaching the forums for FXO voice quality, FXO disconnect supervision, FXO answer supervision to get a sample of some of the problems you are likely to hit. Even if all goes well, FXOs generally have notably lower voice quality.
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PRI if thats on the table. Easy to debug
PRI also gives you the flexibility to provision just the needed lines. like you could probably take a 4 lines on your PRI and just use those channels.
Config wise and debug wise PRI is easier. Personal Troubleshooting experience