Just thought I would add to this to help non technical folks understand IOS updates.
Yes IOS 13.5 has been released and yes the 306 bug is still there.
To understand why you need to understand the “life cycle” of updates.
It takes many man hours of work for developers, programmers, testers and deployment engineers to rollout updates. The release of IOS 13.5 would have taken weeks. It is highly likely version 13.6 is in the testing phase while 13.7 is in the development/ programming phase.
It is unlikely the 306/639 area code error would be caught in testing since it is an obscure error and not been caught previously. To be honest even if was caught it’s unlikely that version would be sent back to be reworked.
Hopefully this bug will get squashed in what ever version is in the programming phase now. Hopefully version 5.7..
Hope this all makes sense. I come from an IT background within SaskTel (now retired so this not an official SaskTel response)