You're looking for something reliable and international - I'd suggest Nexmo (Disclaimer: I do a bit of developer evangelism for them).
- Nexmo can deliver to Spain (along with many other countries)
- Nexmo works directly with carriers, so you have a reduced path of delivery - that makes it cost effective and reliable.
You would send messages with a simple HTTP request, and (although you don't need this now), you receive messages as an HTTP request to your server.
You can even check the delivery stats for a specific destination using the QoD Dashboard, just note that a lower 'Success Ratio' is an indicator of bad numbers, the 'DLR Ratio' is a indicator of how well Nexmo is performing (for example, if you send a message to a invalid number, that effects the success ratio, but not the DLR ratio - since Nexmo will provide a delivery receipt indicating that the message could not be delivered).
