dear you need to know that voice GW ***(VG)*** can't communicate other VG without Call agent involved in this communication and this call agent could be centralized and both VG have connectivity to it through WAN connection for example .

***if your business is small growing*** 

 1. you can use  some sort of of small business call agent (i.e. Cisco call manager express CME) in both sides which can handle Voip calls over WAN and local,mobile and international calls through PSTN  and if you want to obtain small call center it has call center express as well but it is licensed feature. 
 2. you also can use open source call agent like Asterisk and register those GW on it 

***NOTE*** 

 1. the VG register on the call agent by certain registration protocols some of them is open source any one can make use of like H323 and other are proprietary like Cisco MGCP.
 2. i don't aim to mention any product by its name it just for example.