Twin Set configuration sometimes not OK after conversation

[Miraculix01_DUP]

We have an Old 4400 System equipped with IBS DECT with 23 IBS Station ( very good coverage) with SW D1 305 4 ah.
there are installed 153 Mobile 200 / 100 Handsets most of them, are put in Tandem configuration, sometime one or two times per week we have troubel that the release of an call from the handset is not well handled by the system, the Main Number is busy, both extensions ( Fixed and Mobile ) are clearly free !!.

Disconnedtion of the fix set and removing the battery of the mobile fix the problem.

Any idea

Thanks in advance