The Companyflex sip trunk was working fine on your PBX, but recently when you make outgoing calls through the SIP trunk, then received '403 Forbidden' from the SIP provider.
Yeastar S-series PBX
SIP provider did some changes on their side probably after 2022, June.
1. They request the INVITE 'Contact' header should be the SIP trunk Username,
2. They request the INVITE should include 'P Asserted Identity' with the SIP trunk DOD number.
1. Please upgrade the S-series PBX firmware.
a. For S-series V2, please upgrade to 18.104.22.168, then download pbxcenter-s_series-1.18.3-0.7.tar and upload it to the PBX App Center->Upload page
b. For S-series V4, please upgrade to 22.214.171.124, then download pbxcenter-s_series_v4-65.2.3-0.2.tar and upload it to the PBX App Center->Upload page
2. Set the sip trunk number as the DOD number, the DOD number format should be '+(Country Code)(Area Code)(Number)'. Then set the SIP trunk 'Outbound Parameters->P Asserted Identity' with DOD Number.