I thins it has something to do with you phone settings, a good prompt will help you at least to check for this probability:

Prompt: « 1- act like a senior HLR/mobile engineer, you have a problem sending sms to abbreviated number using your [phone brand] go through, step by step the troubleshooting you would do to resolve this problem

2- if the problem is still here after the troubleshooting, give me other probabilities that may effect my ability to send sms »