Dropped Calls with C450IP Handsets

Hi all,

one of our customers is using 4 Siemens C450IP devices connected to asterisk. The 4 handsets are connected to 2 base stations, each base station does have an own repeater. The base stations are configured to use 2 voip accounts, and to assign the matching number to the right handset. The C450IP are updated the last firmware version.

This all does mostly work quite fine - but on about every 5th call - it happens that the call does get interupted after a few seconds, or few minutes.

What i see in the log is - that asterisk seems to no get any more packets from the C450IP - and so it drops the call. An other time i got:

Apr 11 15:43:06 WARNING[5341] chan_sip.c: Maximum retries exceeded on transmission 4253620757@10_0_0_70 for seqno 1071876580 (Critical Response)
Apr 11 15:43:06 WARNING[5341] chan_sip.c: Hanging up call 4253620757@10_0_0_70 - no reply to our critical packet.

Does anyone here have the same problems with the C450IP or an other sip device ?

The C450IP seems to be quite unresponsive the SIP OPTIONS request - it takes the device sometimes about 500ms - 1000ms to answer these requests (qualify).

Any hints would be nice