Problem with greek characters.

I bought the plugin and have set up nexmo. I can successfully send order tracking sms’s but greek characters are shown as ???. I test it with twilio also and there was no problem with the characters. But nexmo states that if there is nothing we can do in our place to change encoding or so. Can you please help me? I want only to use nexmo because of the ability to send viber messages as long as and it is the only service of the 3 that you support. Thank you in advance.