Why does a 304 error occur when sending SMS via C#?

Question

SMS

Please, tell
me I'm trying to send SMS based on this article https://habrahabr.ru/post/256687/
On short messages (up to 70 characters), everything is fine.
On longer ones I get CMS Error 304.
Megaphone modem.
What could be the reason?

Answer 1

https://stackoverflow.com/questions/30358551/cms-e...
Have you read?

Answer 2

Googled a little, here's what I found:
mobiletidings.com/2009/02/18/combining-sms-messages
I haven't tried it myself.