It seems like there are several options, depending on the intended type of site-to-site connection. The 7100 and 1335 could terminate an IPSec VPN tunnel and then you would have a secure, private connection to send voice and corporate data (though voice calls are at the mercy of the Internet unless their Comcast services include Metro E or similar). VPN (or other WAN) would then open the door to integrate phone equipment, place calls between the sites, etc. Do you know if the Vertical IP PBX supports a SIP trunk (like a tie line) to the 7100? That might be one option. If you need to connect to POTS lines at the remote office, then some kind of PBX is required. If the Vertical cannot reasonably peer with the 7100 over SIP, then perhaps another 7100 at the remote office would be the cleanest way to go (with FXO ports for the POTS lines from the Arris gateway).
Do you know how the client would like everything to work, what equipment they prefer to use and what features you need to provide?
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