BLF not working with Snom

[ps1]

we are struggling with BLF and Snom phones working reliable:

- newly started phones: BLF status correct
- call extension: BLF status set correctly
- after some time extension is falsly set to "non available"
- call this extension: BLF again set correctly

any hints where to fix this "timeout" at step 3?

thx
Peter