We are a small company. The main office has two employees. We have a shared main number (faxes and voice). In our business, the fax is used once or twice a month, so we can live with this arrangement. We have a second number in the office, which we use for outgoing calls. It works for us.
Most of the time we know when a fax is coming in from a customer or a supplier. During the course of the week we routinely forward the office phone to one of our cell phones. An unexpected incoming fax can be most disconcerting.
Any kind of unexpected/unwanted fax is a constant bother, regardless of whom it comes from. E-mails can be easily deleted. Anyone who really wants to reach us does so by e-mail. I process 30 to 50 e-mails a day. Probably 10 to 15 are junk e-mails, but with the flick of a key they are deleted. We can live with that.
Sometimes I consider removing the fax completely, but once in a while we need it.