Can the Serial-to-Ethernet device be used the opposite way, i.e. Ethernet-to-Serial? The application is for VoIP devices to operate via an MTS2EA device and a serial modem at each end of a 2 wire leased line.

Yes, the MTS2EA can be used as an Ethernet-to-Serial device and the User Guide provides some examples for this on approximately pages 112 - 124, depending on the revision and under Appendix H! The examples fit all potential usages of the product and do not provide any configuration for the multitude of external ports or equipment.