I'm trying to bridge using a non-registered gateway. And I get MANDATORY_IE_MISSING back. Why is that?

2008-12-15 11:06:49 [DEBUG] switch_core_state_machine.c:152 switch_core_standard_on_execute() sofia/internal/u1000044@192.168.1.112:5070 Execute bridge(sofia/default/0236661201@sip-corporate2.tele2.se)

[.....]

2008-12-15 11:06:49 [DEBUG] sofia.c:2511 sofia_handle_sip_i_state() Channel sofia/internal/0236661201@sip-corporate2.tele2.se entering state [calling]

2008-12-15 11:06:49 [ERR] sofia_reg.c:1312 sofia_reg_handle_sip_r_challenge() No Matching gateway found

2008-12-15 11:06:49 [NOTICE] sofia_reg.c:1333 sofia_reg_handle_sip_r_challenge() Hangup sofia/internal/0236661201@sip-corporate2.tele2.se [CS_CONSUME_MEDIA] [MANDATORY_IE_MISSING]

2008-12-15 11:06:49 [DEBUG] switch_channel.c:1478 switch_channel_perform_hangup() Send signal sofia/internal/0236661201@sip-corporate2.tele2.se [KILL]