Thanks for the details. Like said this board is for Contact Pro product which is a SaaS application and for Contact Centers. We are happy to demo the product if you wish to see it.
Back to your problem. I asked from a colleague and got this:
"I suspect it has to do with them sending with DCS 17. Ask them to retry with DCS 0 for GSM7 encoding, or DCS 8 for USC2, and get back to us if the issue persists."
Now the "us" is referring to support (at) Sinch.com - they are the ones who know SMS service inside out.