capi.conf:

[general]
nationalprefix=0
internationalprefix=00
rxgain=0.8
txgain=0.8

[ISDN1]
msn=17830
incomingmsn=17830
controller=1
group=1
softdtmf=on
context=extern_isdn
devices=2
immediate=no
language=de

extensions.conf:

[general]
static=yes
writeprotect=no
TRUNK=CAPI

[globals]

[default]

[extern]

[extern_isdn]
include => extern
exten => 17830,1,SetLanguage(de)
exten => 17830,2,Set(CALLERID(num)=1234567)
exten => 17830,3,Dial(SIP/22&SIP/23&SIP/24&SIP/25&SIP/26,20,o)
exten => 17830,4,Hangup()

sip.conf

[general]
port=5060
context=default
disallow=all
allow=ulaw
Localnet=198.0.10.0/24
dtmfmode=rfc2833
language=de

[22]
type=friend
fromuser=22
username=22
secret=*******
host=dynamic
context=intern_sip_22

[23] to [26] are following with every 22 replaced

debug of call:

Asterisk 1.2.1, Copyright (C) 1999 - 2005 Digium.
Written by Mark Spencer <markster@digium.com>
=========================================================================
Connected to Asterisk 1.2.1 currently running on arthur (pid = 25708)
Verbosity was 0 and is now 9
  == ISDN1: Incoming call 'xxxxx481350' -> '17830'
    -- Executing SetLanguage("CAPI/ISDN1/17830-0", "de") in new stack
Jan 27 18:53:13 WARNING[26368]: pbx.c:5435 pbx_builtin_setlanguage: SetLanguage is deprecated, please use Set(LANGUAGE()=language) instead.
    -- Executing Set("CAPI/ISDN1/17830-0", "CALLERID(num)=1234567") in new stack
    -- Executing Dial("CAPI/ISDN1/17830-0", "SIP/22&SIP/23&SIP/24&SIP/25&SIP/26|20|o") in new stack
    -- Called 22
    -- Called 23
    -- Called 24
    -- Called 25
    -- Called 26
    -- SIP/23-d2a8 is ringing
    -- SIP/24-fced is ringing
    -- SIP/25-3051 is ringing
    -- SIP/26-80aa is ringing
    -- SIP/22-aed0 is ringing
       > CAPI INFO 0x3490: Normal call clearing
  == Spawn extension (extern_isdn, 17830, 3) exited non-zero on 'CAPI/ISDN1/17830-0'
  == ISDN1: CAPI Hangingup

system:

debian testing with some stable packets
Asterisk 1.2.1