You could install Asterisk/FreeSwitch/FreePBX internally and purchase a SIP trunk with unlimited minutes from a provider like Aretta (https://www.aretta.com). I personally don't have any experience with them.
Or, you could pay to have the provider host your PBX, which will probably save you a ton of work, but will (of course) cost more.
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There are so many options now.
You could install Asterisk/FreeSwitch/FreePBX internally and purchase a SIP trunk with unlimited minutes from a provider like Aretta (https://www.aretta.com). I personally don't have any experience with them.
Or, you could pay to have the provider host your PBX, which will probably save you a ton of work, but will (of course) cost more.
http://www.phonebooth.com is another option.