Where is the caller being called in the message? The provider says that it should be specified after INVITE (044123456@). The developer of our PBX insists that the called subscriber should be specified only in the TO field (431234567@). Who is right?
09:46:21:643 4036 RECV -- UDP 963 Bytes FROM *.*.*.*:5060 IFACE *.*.*.*:5060
INVITE sip:044123456@*.*.*.*:5060;user=phone SIP/2.0
Via: SIP/2.0/UDP *.*.*.*:5060;branch=z9hG4bKvspa8h207ot1h01df1g1.1
Call-ID: SDhp7lb01-45b016634f16c79e5f1346503fe94984-ct72062
From: <sip:056123456@*.*.*.*;user=phone>;tag=SDhp7lb01-ltm9m107-CC-28
To: <sip:431234567@*.*.*.*;user=phone>
The callee is dictated by the request URI so in your example it's sip:044123456@*.*.*.*:5060;user=phone.
The To header indicate the logical recipient. It can be different from the request URI and is not used to determine how a request gets routed.
Based on the information you've provided your provider is right and your PBX developer is wrong.