I get a cell phone and buy the call forward service of Bell. ($2/month). When my phone line is busy or not answered, calls will be forwarded to my cell phone. This way even if I am online, I have the choice of picking up the phone instead of listening to message later. If I don't pick up my cell, the caller can leave a message. No charge for this service, i just need to pay for the air time when i check the message.
If you drive, you need a cell phone for emergency anyway, and the handset is almost free. If you don't use it a lot, just get a pay -as-you-go option, like the Fidomatic plan, instead of paying for a $20 month plan or above.