Two identities from same phone registered on same asterisk


can someone please help me with the following:
I have a Snom phone registered on one asterisk (1.4.18) server having two indentities (private/business).
Based on the selected identity on the phone, I set the outgoing CallerID to my private or my business number.
Asterisk always chose the same identity (phone CLID). So it seems to have a problem with these two identities.
A shows both identities on the same phone:

31/31                    D          2051     Unmonitored
32/32                    D          2051     Unmonitored

When I do a sip debug, I see the incoming INVITE from the selected identity (31):

INVITE sip:33@ SIP/2.0
Via: SIP/2.0/UDP;branch=z9hG4bK-a7ayq8m0xy3o;rport
From: "31 (Till)" <sip:31@>;tag=te6w3uswji

But the session is connected to the identity 32:

Spawn extension (macro-stdexten, s, 1) exited non-zero on 'SIP/32-0a0f9258' in macro 'stdexten'

I don’t have a clue how to solve that.
Any help is appreciated!