There must be something very wrong with the way the Siemens OpenScape Office PBX's SIP stack handles SIP error codes or the way people configure it by default.
Suppose you make a call and forget a digit somewhere in the middle. Your PSTN provider (or someone else) detects that the number is not quite sufficient and replies to your INVITE with an error message containing "SIP/2.0 484 Address Incomplete".
Normal SIP implementations send an error up the stack to the end user who then hears some kind of error code through their handset or some error code on the display of their phone. Is OpenScape doing that? Noooooooo.....
When OpenScape receives an error code other than "486 Busy" or "500 Internal Server Error", it sends out as many retry calls as it can, one after the other, until it hits the maximum number of simultaneous calls it has configured on the SIP trunk (even though these calls are not simultaneous).
If you're using the distribution between successful and failed calls to monitor the health of your platform, behavour like this very quickly starts to trigger alarms that something is not quite right, especially when endusers are absolutely convinced the number they dialled is correct. Hundreds of failed calls within a minute or two are not unusual in cases like this.