You need to have a gatekeeper, not a gateway.
The gateway is a termi nating device, one that will get VOIP and pass it to the PSTN or phone lines.
The gatekeeper is a device theat will receive the registration from one ATA and tell the other ATA ‘s where to send the calls.
Get the IOS from the Cisco router. Go to the Cisco site and find out if your IOS supports gatekeeper.
Normally, the small units do not support gatekeeper.
I think that the smallest cisco router that supports gatekeeper is the 2600 series.