This number (123)-456-7890 is also a "generic" caller ID number provided by the Skype software when you're being phoned by someone using Skype. This is problematic, because while it's possible that a real friend or associate is trying to call you from somewhere in the world using the 'Net and Skype, it's also become increasingly common for telemarketers and scam artists to "hide" their real identities using Skype rather than the regular phone network. (They also like Skype because all their calls are flat-rate, whereas they may have to pay on a per-call basis for the use of the POTS network.)
I never answer. If it's someone I know, they'll leave a message.
By the by, because of this generic caller ID, Skype's becoming increasingly popular with pranksters and a**holes. I got one Skype call around Christmas last year that consisted of some college student and his buddies berating me over the phone for the better part of several minutes (it was on my voice mail). They phoned around 3 A.M.