That would depend on the situation, but for a small business with VoIP phones, FreePBX works just fine. At the bunker (the previous VHS location), we had a setup with a VoIP provider, FreePBX, and multiple VoIP phones through the space (that all rang when there was a call).
For PotS, you can get Digium FXO cards if you need to hook up land lines, and FreePBX even supports IVR applications.
I follow a YouTube channel “Crosstalk Solutions” he has a good series on FreePBX - though there is a newer version since this series the basics are the same