Raoul's reply speaks to why I didn't go into more detail, since there are a hundred things we don't know about your network and how it works. If you can turn off DHCP on your router that is currently providing DHCP and the IP Office can provide DHCP for those devices, everything should be fine with just the IP Office providing DHCP. You'll want to be sure that the scope for the IP Office isn't trying to hand out addresses that might be set statically or conflict with existing leases. Then everything will work for you.
The other alternative is to setup your existing router with the 176 and 242 options as I mentioned before. If your existing router can't do that, then best to use the IP Office. However, if your router needs to hand out addresses for VPNs and other things you might have problems.
The way we setup for all of our clients is we have the router or a server handing out addresses on the main vlan and the router or IP Office handing out addresses on their VLAN, but we only ever use the one NIC on the IP Office. You would need to have switches that have the VLANs setup on them and move the phones and IP Office to another IP subnet with the proper routing, etc.
I thought, from your post, that this was new to you. From your second post that is confirmed, so I am trying to present the simplest possible way to accomplish your goals without your having to become a networking expert.
Easiest is just use the IP Office for DHCP and turn off DHCP on your router if possible. Particularly if this is a small (under 100 device) network.