I don't know from Elk standalone, but if you integrate the Elk & (HA software package of choice), you can do really cool stuff like record a different .wav file for each friend or family member, and announce that over the Elk speakers when the phone rings.
In my house, I recorded my M.I.L. saying "Hi, it's me, where are you guys" in that sarcastic tone she has, and that's what gets played. My wife's actor friend who's funny as hell but just a little different from everyone else just re-recorded his to be "Farty farty fart. The farty flintstone is calling you. Pick up the phone.".
Not only is it much faster than a generic TTS or speaking the #s, it's the type of thing that the kids (& me) crack up at literally every single time he calls, and doesn't get old :lol