Why there is error 304 when sending SMS via C#?

Please tell me
Trying to send SMS on the basis of this article https://habrahabr.ru/post/256687/
For short messages (up to 70 characters) all the rules.
On a longer get a CMS Error 304.
The Modem The Megaphone.
What could be the reason?
June 10th 19 at 15:57
2 answers
June 10th 19 at 15:59
Of course, I'm not lazy to Google this solution tried. - brendon78 commented on June 10th 19 at 16:02
June 10th 19 at 16:01
I googled a bit, found this:

Not tried it myself.

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