[asterisk-r2] problems with digium mfcr2 and Siemens EWSD

[Laura]

Hi,
I have a digium E1 connected to a Siemens EWSD switch, although the 
comunications are established the EWSD gives a "failure of pcm alarm". 
I'm using dahdi and openr2

This is my system.conf
span=1,1,0,cas,hdb3
echocanceller=mg2,1-15,17-31
cas=1-15:1101
cas=17-31:1101
dchan=16
loadzone=ar
defaultzone=ar

chan_dahdi.conf
[channels]
echocancel = no
rxgain = 0.0
txgain = 0.0
signalling = mfcr2
mfcr2_variant = ar
mfcr2_get_ani_first=yes
mfcr2_max_ani = 10
mfcr2_category = national_subscriber
mfcr2_logdir=span1
mfcr2_logging=all
;mfcr2_mfback_timeout=5000
mfcr2_allow_collect_calls=no
mfcr2_double_answer=no
mfcr2_immediate_accept=no
mfcr2_advanced_protocol_file = /etc/asterisk/r2proto.conf
mfcr2_forced_release=no
mfcr2_charge_calls=yes
mfcr2_max_dnis = 20

group=1
context=ir2-incoming
channel=>1-15

group=2
context=or2-outgoing
channel=>17-31

This configuration works when the digium is connected to a Siemens SDE o 
to a Lucent switch.

I've made lots of changes to my system.conf :
span=1,1,0,cas,hdb3,crc4 (the digium is constantly recovering from blue 
alarm)
I've also changed timing source to 0, and cas bits to 1001, 1000 and all 
possible combinations but nothing works.
¿Any ideas?
Laura