Cisco 7975 won't register to trixbox?

Hi,

I've been struggling for a long time to get Cisco 7975 to work. It is now semi-working. I managed so far to accomplish:
1. Display the correct extension and name on the phone
2. Display the correct date/time
3. Make outgoing calls.

Currently the phone is able to call other phones on the network (the other phones are Cisco 7960) and it is also able to dial out.
However when one of the phones tries to call the 7975 it displays "Reorder" on the screen and it won't call it. Also when there is an outside incomming call it doesn't ring on this phone.
Looking at the log file it seems that the 7975 is not registering with trixbox at all.
After restaring trixbox at the end of the log I get:
[Nov 14 15:39:43] VERBOSE[22728] logger.c: Asterisk Ready.
[Nov 14 15:39:43] NOTICE[1918] chan_sip.c: Peer '102' is now Reachable. (138ms / 2000ms)
[Nov 14 15:39:43] NOTICE[1918] chan_sip.c: Peer '103' is now Reachable. (241ms / 2000ms)

102 & 103 are the Cisco 7960 and the 7975 is 101, but it never gets registered. In the trixbox status it also says 2/3 phones are connected.

When I try to call the 7975 from any of the other phones I get in the log file the following:
[Nov 14 15:10:09] VERBOSE[26411] logger.c: -- Executing [s@macro-dial:7] Dial("SIP/101-08294330", "SIP/101||tr") in new stack
[Nov 14 15:10:09] WARNING[26411] app_dial.c: Unable to create channel of type 'SIP' (cause 3 - No route to destination)
[Nov 14 15:10:09] VERBOSE[26411] logger.c: == Everyone is busy/congested at this time (1:0/0/1)

I don't understand how is it possible that the phone can dial out and dial in the local network, but cannot receive calls?

The SIP firmware on the 7975 is: SIP75.9-2-1SR2S
Boot Load ID: tnp75.8-3-4-16BN01.bin

Pleae help me on that as I've been struggling with this for ages.