Weid provider... Need to change Contact and From header independently

Inclined Readers,

Strange Problem or better weird carrier.
For outgoing calls, they expect the contact header to always be +E164 without outgoing extension, while the FROM header SHOULD be +E164 PLUS the outgoing extension, in order to correctly set the outgoing callerID.
I’ve been searching for almost 10hrs to find a configuration for this, but I can’t find any suitable solution…
Except one topic from 2016 here where it’s said this cannot be done: Asterisk sip Contact change

I also found this, but that didn’t work either:
To my understanding of SIP, this is not exactly in accordance with RFCs anyway.
I can set fromuser in sip.conf, but this of course also sets the FROM header, preventing me from sending the originating extension using CALLERID(NUM). So I end up with only the trunk number as oputgoing Caller-ID…
I am using chan_sip (chan_pjsip out of question at the moment), asterisk is 13.14.1 standard Debian package
My question: Can you people confirm that it’s really not doable (before I spend some more hours ruffling my hair), or does anyone have an idea how this could be helped? The only option I have is to buiy the CliüpNoScreening Option for €29/Month which the customer of course is reluctant to do….
Ah, yeah, just FYI: You will run into this, when needing to use A1 Telekom Austria as your SIP-provider….

Another reason to use the PJSIP stack. BTW, German Telekom has similar requirements for its products.

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