و عليكم السلام و رحمة الله و بركاته ..
You will need the following :
1- IOS Image for 3745 Router with the following Feature : CME ( Call Manager Express ) ... you will need to get the ios from the internet . i recommend the following feature sit : IP-IPGW
2- You will have to configure Cloud in the GNS to be bridged to your Ethernet connection to reach the LAN which includes the 2 IP Communicators .
Please tell me if you have any problem with that .