IPphone Strange Issue

Hi everyone,
I have implemented asterisk 1.4.21 in my corporation and I’m experiencing some strange behavior on my Ipphones. A few minutes after registration, they become inaccessible randomly. I can dial numbers on them, but no one can call the phones back. In the asterisk CLI, I receive following errors:

  1. Calling SNOM® 300 IPphone:
    logger.c: – SIP/5050-0b1495b0 is circuit-busy
  2. Calling D-Link®DPH 140s IPphone:
    logger.c: – Got SIP response 505 “Version Not Supported” back from
    Please mention that I have tested both models in another implementation and they work like a charm.
    I have been trying to solve the problem for two weeks and I have replaced whatever you can imagine on my network. I have reinstalled Asterisk twice, reconfigured my Cisco based network, changed my Ipphones with new ones, used another codec and so many other things. The last thing I am suspected to is that Asterisk is not capable to register my 80 user properly, because I have three other Asterisk implementations and they have no problem at all.

Thanks in advance.