The Easybell sip trunk was working fine on your PBX, but recently when you make outgoing calls through the SIP trunk, then received '407 Proxy Authentication Required' from the SIP provider, and at your IP phone got error '603 Decline'
Yeastar P-series PBX
Our PBX sent INVITE with Authorization, but Easybell side still replied with 407 Proxy Authentication Required. It seems they changed authentication of From Header.
Please set the SIP trunk Outbound Parameters as below, and then please set the Outbound caller ID name as the Outbound caller ID for extensions under the Outbound Caller ID page.