On the 2nd gen Total Access 900e series, T1 0/1-2 are used for carrier/WAN while T1 0/3-4 may be used for PRI. The TA900 series have a single T1 WAN and single PRI interface.
My experience with fax over FXS on the TA is positive. The reliability will have everything to do with the carrier/trunk side quality of service.
My understanding of the resource limitations is that some of the 2nd gen units have limited DSP for IP-TDM sessions (a TA908e model, I believe). But I think you may use any number of TDM-TDM channels up to the physical interface limit.
Thank you, Chris
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