Unless the 3120 must be used for some reason, I would consider using the 1335 for all functions. The 1335 is a full-blown router, firewall and VPN appliance with enhanced features such as Voice Quality Monitoring (VQM). The 1335 may not be a super-powerful unit by today's standards, as far as routing performance, but it will run circles around the 3120 which was really designed for routing loads at DSL speeds years ago. Also, the 1335 includes limited Layer 3 Lite route-caching which can route (switch) traffic between VLANs at full wire speed.
Either way, it's commonly recommended to configure separate VLANs for default data and voice. This way you will segment your devices and applications to protect from trouble in one network from degrading traffic in the other. Also, you will have flexibility to use different DHCP options and other aspects to fit the needs of your voice endpoints. VLAN is a Layer 2, logical segregation of devices and traffic. Your 1335 (and/or 3120) must have an IP interface within each VLAN to be the default gateway for that subnet and also provide for inter-VLAN routing. The guide The difference between VLANs and VLAN interfaces is an excellent resource.
SIP proxy is used when the 1335 or 3120 should monitor SIP devices and sessions, typically used to take over SIP call setup when a site has lost connection to the PBX. This is mostly used at a remote site where there is no local PBX and the ADTRAN SIP proxy can handle call setup between extensions at the site or route external calls to a survivable gateway connected to one or more local POTS or PRI lines.
Let us know if you have additional questions. If you get to a point in your configuration to share it, remember to remove passwords before you post it here. A diagram may be helpful if you need assistance.
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