I was wondering the same thing a while back. I really thought it would be more professional to have a land line. So what I did is get a bt line installed with call diversion. I have all calls diverted from my land line to my mobile currently. But in the future if I decide to get an office instead of running it from home I will already have the number in place. I also have the VOIP that was installed with the land line because I took the internet option also. I use the VOIP as a dedicated fax line so I have the appearance of a hard office even though I am still working from home. Not that I think working from home is a bad thing, it sure saves on overhead and I believe I will be able to work from home until about the 250,000 to 300,000 turnover mark, but at that time the cost of the office will be spread over more contracts and be more viable without having to increase prices and make my business less competitive.
Long story Long, I think having a landline looks more professional but saying that I made it to six figures without the landline. I think it is something to consider if you have plans to move to the next level and if your margins allow it, it is better to get it before you need it so that it is in place at the right time. I think contracts of 1000 per month or larger probably would consider what they percieve as proffessioalism, could be land lines, fax lines, lettered vans, uniformed staff, ability to provide consumables.
Cheers
Gilbert