SNOM 7 series loses registrar

[sanderkoers]

we upgraded from grandstream to SNOM 765 and 745 phones.

only they seem to lose registration on various occasions, reducing te re registrar time helps but not enough.
Phone says; not registrered

any ideas ?

 

[sanderkoers]

The phone is registred until someone transfers an outside call to my phone, when i accept the call everything keeps working and i can finish the call (but the screen already says NOT REGISTRED) after i end the call i need to re-register the phone to accept new calls.