I think people would like to know if the user is in DND mode or if their call will go straight to voicemail. In fact, having an option to leave a voicemail instead of calling the user in realtime might even be something that some users want.

I don't like being disturbed while coding, but I wouldn't mind checking my virtual voicemail now and then.

Re: Pricing

Also, I do really enjoy the up-front per-call pricing model. It's easier than a flat monthly rate and per-minute charge typically associated with the VoIP industry.

I can understand how it'll make your finances risky though. It's likely just a matter of time until some premium rate scammers start knocking on your service to see if they can make 12-hour-long calls.

If Twilio supports it, maybe you can put a time restriction in place? Or make it so a call that consumes a call credit every 5 minutes.

Also, Local US rates with Twilio are pretty much 1.5 cents a minute. The international calls get more expensive, but maybe you could do it so more expensive calls use more call credits.

Some things you might want to put in a FAQ or on the pricing page:

1. If leaving a voicemail counts towards the call credits.

2. What happens when a user runs out of call credits (does every contact just go to voicemail?)

3. If there are any restrictions on voicemails (auto delete after X days, for instance - Something you should consider. 1/4th of a cent per minute and 1/20th of a cent per month for storage can add up if you get big)