<b>Our Scenario:
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A fax located in a non-T.38 SIP VoIP network (non-Cisco) is calling a fax connected in a PABX connected to the SIP network via a 26XX PRI-VoIP Cisco router with IOS 12.3.(e.g. 3640 Cisco Router)
The call starts from the network with default compression codec (G.729), fax should be normally be detected by 26XX & change to G.711 should follow.
However it is required that 26XX informs via SIP signaling the SIP network for this change. Normally the SIP-network expects a re-Invite so that it adapts to the new SDP.
<b>Our Question:
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If re-invite message is sent & with what SDP from the 26XX.
Also in the other way around i.e for Fax calls originating from PABX-26XX towards the SIP network : if Cisco 26XX will accept SIP re-invite so that to switch to G.711 pass-though via SIP signaling.