Twilio has a pay-as-you-go model, but offers a free trial credit to start with.
Telnyx offers free inbound SMS texts, but charges $0.0045 Per Outbound Message.
Plivo is another SMS service that has pay as you go pricing, but offers free trial credit.
Messagebird, like Telynx, offers free inbound SMS texts, but charges for outbound SMS messages.
ClickSend inbound SMS messages are free, but outbound SMS messages depend on volume.
|2k or more||$0.0088|
|10k or more||$0.0073|
|100k or more||$0.0067|
|Over 200k||Custom Pricing|
An SMS gateway can be set up to handle the connections to the SMSCs. Now the SMS text messaging application only needs to know how to connect to the SMS gateway. To support more SMSCs, you just need to modify the settings of the SMS gateway. No change to the source code of the SMS text messaging application is required. The use of an SMS gateway can greatly shorten the SMS text messaging application's development time. Source: developershome