Of the 4 VOIP ATA's I've used, they all support key system trunks with a REN of 0.4B or whatever. It seems that somewhere I read an ATA is good for 2 REN's. Even that would be 5 0.4 devices bridged on the line, but the key would be that you don't even attempt to do that.
I guess that means it's fully POTS compatible, but not more than a couple of POTS devices. Just pretend you live in the sticks.
As far as number of lines, we stacked up 4 Vonage lines in a poor man's hunt group using CF-Busy feature on lines AB and C. The nicest part of that is we could build it and tear it down via the web. Eventually, we did a CF-Busy/No answer to the customer's main land line since we didn't give those number out to the general public and the outbound access was via a group CO key.