Is there a way to have multiple telephone number?

There's no easy way (through cell phone companies) to use multiple phone numbers for the same cell phone. I know for a fact Verizon Wireless doesn't offer vanity numbers, but I'm not sure about the other providers.

Services like GrandCentral (http://www.grandcentral.com/) allow you to get another phone number and forward calls to your phone and "mask" outgoing calls so it appears to have come from your GrandCentral number instead of your cell phone number. It's only in beta though and it's difficult to get an account right now.

There's also Skype. You can get an "Online Number" from them. http://www.skype.com/allfeatures/onlinenumber/ -- not an expert on this, but there should be some way to do call forwarding to your cell phone, but you might need a subscription.

you could set up an asterisk server and set up as many extensions as needed. the number of simultaneous calls is limited by the number of phone lines. there is also a way to connect skype to asterisk, but i've never done it.

We've been using http://ringcentral.com/

Its a great service and you can add additional #'s to your account too

GrandCentral (grandcentral.com) allows you to get a second number that can forward to any phone, and you can selectively forward, block numbers, and use a whole host of other features.

Brring! (brring.com) allows you to get a second number that forwards to your phone, and plays 10-seconds ads that you get paid for.

I use Vonage and you can have has many "virtual" numbers that you want. They can be in other cities even other countries and they will all ring to the same phone. You can operate from your basement and give the impression that you are a huge corporation with offices all around the world and country. Most VOIP service provider will give that kind of service and even more.

my company has a T1 with TDM voice channels on it, feeding into asterisk. (hot tip: buy the digium hardware) we get new phone numbers routed to the T1 for $25 activation and, uh, $0.15/month. (couple hundred a month for the line itself.)

as for multiple phone numbers routed to POTS lines, i don't see why your phone company couldn't do it. you just wouldn't be able to tell which line the call was for.

In the trade, phone numbers are called DIDs. Once you realize that you want to buy 100 DIDs you should have an easier time finding providers. http://www.voip-info.org/wiki/view/DID+Service+Providers

What is it good for? Honestly.

DIDs cost money, not a lot if you buy them wholesale, but still require a monthly payment.