[Serusers] SER & PSTN gateways (Ovislink?)

Is anyone using any of the Ovislink PSTN gateways (eLive 400/800) with SER? The products specs state support for H.323 and SIP. Like many products it means that the unit supports only H.323, and you need to download new firmware which replaces H.323 with SIP. I'm new to SIP in general, and haven't got a working configuration yet. How is PSTN gateway security supposed to work? I see in the manual there is an example using SER to protect a "simple" gateway. Ovislink definitely fits into that category. I guess that means that I need to put ACLs on the router to make sure that all SIP traffic to the gateway is blocked except from the SER server. How do better gateways implement security? Any nice PSTN gateways around that anyone can recommend for use? I'm looking at Cisco, which seems like a lot of people use. I've looked at Patton and Vegastream too. Neither of these companies has a 8 port POTS PSTN gateway though. Tom