Overview

You may face a high rate of SUB busy error with another operator's home-routed MT-MT SMS traffic, which may be reporting subscriber busy responses from your side.

Information

The incoming SRI-SM is received by one router (e.g. RTR-01), which replies the scrambled IMSI and its own GT to get the MT message back.

The MT message is a long SMS and all the segments should be routed to RTR-01 but one of them is routed to, e.g.,  RTR-02 instead. Both parts are delivered about the same time but the MT sent by RTR-02 gets MT subscriber busy, as RTR-01 was delivering the MT with the flag moreMessagesToSend(5) = true(1).

It's recommended to create an MTIR rule to store the incoming MT message on AMS before delivery.

That way, AMS can deliver all segments in sequence, preventing the subscriber busy situation.

To investigate what might be causing the message to go through different RTRs, please refer to the related troubleshooting article.