In the Mendix world App Services demo we used https://www.nexmo.com/
We used https://www.messagebird.com/. This service provides SMS inbound and SMS outbound. This one works pretty simple and there is a trail version available.
We use Clickatell. They give you a free API and a couple of credits when you sign up. Very easy to use and to implement. You can send/receive and also loads of other stuff:
Flash (Popup) messaging *
Multi-language unicode messaging *
WAP Push messages * MMS Push messages * Binary messaging *
Specify message prioritization (queueing)
Custom sender ID *
To-the-minute scheduled delivery (Up to 7 Days in advance)
Specify message validity
Specify your own message IDs
Control cost: specify maximum credit charge per message Query coverage availability before sending
Stop/Delete a message
Query credit balance
Query individual message status Push message status notification and cost (Callback)
Query individual message charge Batch/Bulk Sending including custom fields
Specify required features
There are many SMS providers in the market who can provide you with SMS services but I would suggest you be careful where you invest your money as there is a lot of Fly-By-Night Bulk SMS provider in the market. I would suggest you check out SMSala platform they have the highest delivery rates and are presently the No.1 SMS platform in India and UK.
I have used https://www.twilio.com/ for a personal project.
They have a queue system if you want to send a lot of SMS
They have a lot of known clients in their portfolio