Handheld does not receive incoming calls

[dunzi]

Hi all,
I have a weird problem with my handheld.
It does not receive any incoming calls. It just shortly beeps and then I have a missed call on the screen.
Paging from the base station works and a test ring from my ISP router does work as well. Outgoing calls work.
I have resetted both base and handheld and upgraded to latest firmware but nothing helped so far.

DnD is off, Call forwarding is off,

Don't know what else to check.

Best regards
Jan