phonemail network from/to US and Mexico

Installing new Siemens Hicom system in Mexico
with phonemail. Networked via a E-1/T-1 (WAN)
over a Cisco 3810 running voice, data, video.
How can we design Hicom in Mexico under int’l dialing sequence to be “transparent” and talk (receive/send network messages) between Mexico and the other plants in the USA which are Rolm 9751-mod 10’s, and the
headquarters in the USA which is a Siemens Hicom also???