We got by with an answering machine for six months, but people resent it. You especially lose out on new signups with an answering machine. They just tend to call the next number on their list. We get most of our business from that and from word of mouth. We did a few weeks of radio advertising but it was very expensive.
Untargeted advertising can be tricky.
Billboards, radio, and general newspaper ads get accounts, but not as high a percentage as targeted magazines. However, doing big advertising is probably good in the long run as it earns name recognition. Trade as much as possible. Our CPA gets a free ethernet link. We trade accounts for pagers.