I have been using Twilio with FreePBX for however it was that I first joined this form. Have also used inbound Twilio for 2 years to ACME packet SBC's for inbound. Have ported customers to them every month outside of our local market (away form Level 3 and Verizon) and by then end of the year all our extended area inbound will be twilio. We have a cross connect with them but OTT worked flawlessly.
Have never had an outage. Best kept secret around.
It is 6/6 billing, even when you are 10mmou. Its 60/60 for inbound and outbound. For your avergae single account 6/6 only nets you 12% savings, where there rates are 55% cheaper on inbound
Outbound subaccount calling is difficult to configure. Inbound requires you peer to all of the market IP addresses in freePBX separately.
But it works great.
EDIT: If you are going to use them I suggest Telnyx for outbound. Its 6/6 pricing for .0075 for outbound. Its also easier to configure than outbound on twilio sub accounts (assuming youare reselling)