Have a couple people we have sent documents to sign, and they put in their cell number, and get a message that says Error:  Unable to send the SMS, please contact the sender of the document.

Does anyone know what would cause this, and what needs to be done to resolve it?  Could it be carrier based, though that I would think would be unlikely or this would be a bigger issue, and I would have probably found a question about this having already been posted.  

This is using the latest 1-app free version.  Is there a limit to the number of documents that can be signed with this version?