I have offered phone services in the past but it's not my favourite thing. As Mary says, not everyone enjoys doing it. But I can give you some suggestions based on experience.

1. Think carefully about whether you want to be tied to your office all day, every day. If you are providing phone services for other people the expectation will be that you are available all, or most of the time.
2. If you're going to do it for multiple clients, then look into services where you can have several lines installed in your office. I have a mult handset, i.e. it has 4 lines plugged into it without me having to get special hardware installed in the office. This is the one I have (the black one) so perhaps there's something like this where you are.
http://www.drl.com.au/details.asp?It...tID=0&CatID=18
3. Make sure you have a dedicated voicemail message for each line for times when you can't answer the call - you might be on another line, or just out of the office, or it could be when you're in bed at night.
4. Look at your costs - I charged on a per call basis but did my homework and soon worked out that if clients had more than 75 calls a month, it would be cheaper for them to go to a serviced office instead.
5. I recorded all calls received into a spreadsheet and emailed them to clients at the end of each day. These days with text messages clients might prefer to have urgent ones sent to them by that medium.

There are probably other aspects to this but these are the things that I found, with number 1 being the most important. I did not want to be tied down to my office so the lines I have in my office currently are for small businesses that don't have a lot of calls and do not expect me to be here 18/7.