Thanks for the info! How much would the cell phone plan cost that allows unlimited sms messaging?

Is there any reason to use twilio over something like this? Twilio costs 0.0075 / message

Choosing between twilio or this definitely depends on your use case. Gosms is handy when you have no internet connection, you require total control over the infrastructure and such. Use twilio when you don't want to worry about anything and just send messages. There could be more scenarios too.

Twilio's pricing schemes change per country, for us in India (where 1 Rupee = 100 Paisa and USD 1 = ~62 Rupees), twilio charges 60 paisa per message. At the same time, some tel-com providers have plans that cost only 5 paisa per message. India has other limits and regulations but for personal/in-house use they hardly matter.

To add to this, If you want to send fast worldwide SMS reliably and without much hassle, use twilio or plivo or other such web sms gateways. If you want to send SMS locally, for cheap and willing to hack your way around, give GoSMS a shot. Given the time a modem takes to send an SMS, with a single modem connected, the rate of sending SMS is low (~1 message / 5 second).

Many "Unlimited" providers would be unhappy if you were sending 10s of messages per second for sustained periods of times.

Ting charges $6/device plus $0.0025 per message.

Depends on the provider. Thirty bucks? But you'd better make sure their "unlimited" means "unlimited".