Can I keep / use my existing DIDs/ Voip Provider? 

No, Twilio is the preferred provider at this time.  

Are there any other fees per month besides the cost of seats? 

No, you only pay for the seat rental and telco costs.

What message delivery options are available? 

All standard methods are currently available, Email, Fax and SMS Text.  Currently we are developing SMS with reply, where your clients can txt reply message receipt.