Outgoing call with hangup 27 on dahdi_trunk - incoming calls work

Hello, I’m using a DAHDI trunk in my PBX. Inbound calls work in general and are accepted by the PBX. But outbound calls just give my “all circuits are busy now”.
The number is 100% correct and is a regular german mobile number that can be dialed from any normal landline using 0173xxxxxxx.

This is the outbound route I use:

[outrt-1] ; 0_out
exten => _0.,1,Macro(user-callerid,LIMIT,EXTERNAL,)
exten => _0.,n,ExecIf($[ "${CALLEE_ACCOUNCODE}" != "" ] ?Set(CDR(accountcode)=${CALLEE_ACCOUNCODE}))
exten => _0.,n,Set(MOHCLASS=${IF($["${MOHCLASS}"=""]?default:${MOHCLASS})})
exten => _0.,n,Set(_NODEST=)
exten => _0.,n,Gosub(sub-record-check,s,1(out,${EXTEN},))
exten => _0.,n,Macro(dialout-trunk,1,${EXTEN:1},,off)
exten => _0.,n,Macro(outisbusy,)

This is a log snippet when dialing out (I dial 00173xxxxxxx from the internal device as seen in the outbound route dialplan):

pbx.c:     -- Executing [[email protected]:22] Dial("SIP/89-00000018", "DAHDI/g0/0173xxxxxxx,300,Tt") in new stack
sig_pri.c:     -- Requested transfer capability: 0x00 - SPEECH
app_dial.c:     -- Called DAHDI/g0/0173xxxxxxx
sig_pri.c:     -- Span 1: Channel 0/1 got hangup, cause 27
app_dial.c:     -- DAHDI/i1/0173xxxxxxx-5 is circuit-busy
chan_dahdi.c:     -- Hungup 'DAHDI/i1/0173xxxxxxx-5'
app_dial.c:   == Everyone is busy/congested at this time (1:0/1/0)
pbx.c:     -- Executing [[email protected]:23] NoOp("SIP/89-00000018", "Dial failed for some reason with DIALSTATUS = CONGESTION and HANGUPCAUSE = 27") in new stack

Someone has an idea what is configured wrong here? Incoming calls using that trunk are ok.

Code No. 27 - destination out of order.
This cause indicates that the destination indicated by the user cannot be reached because the interface to the destination is not functioning correctly. The term “not functioning correctly” indicates that a signal message was unable to be delivered to the remote party; e.g., a physical layer or data link layer failure at the remote party or user equipment off-line