It might seem like common sense, but if you want clients (especially potential clients) and others to contact you, make sure they can.
A colleague recently related his experience trying to talk to a commercial real estate agent about a retail property. The big poster on the shop window clearly indicated a cell phone number for the agent.
A call to that number rang several times before an answer service advised callers to ring the company office. On ringing the company office, another answer service immediately suggested ringing the agent if they wanted information.
The potential for a missed business opportunity is huge, as is the likelihood that this caller will tell others about his bad experience. It reflects badly not only on the agent, but also the company he represents.