SMS - Recieving SMS to my web app ~ what do I need?

I am a PHP/MySQL app developer and I am researching the feasibility of taking info via cell phone SMS and using that info to trigger a web site posting. I can handle all of the web stuff, but I know nothing of and cannot find much info on the SMS part.

Here are my requirements/unanswered questions:

- I am limiting my area to USA + possibly Canada
- I would prefer that users don't have to enter an email address, but preferably a phone number (service provider independent)
- I am open in regards to using a provider/service as a gateway, but I do expect to receive tons of SMSs so it has to be cost effective
- The only outgoing SMS would be a confirmation type set up (I am not interested in SMS marketing campaigns)


Here is a basic outline of my app:

This site will be a sort of listing service. Visitors (surfers) will be presented with a list of service advertisements which is updated every 15 minutes. Advertisers (users) will be able to announce their availability through this service. I want my advertisers to be able to post an ad via cell phone or via the web.

Any help or suggestions on this matter will be greatly appreciated! TIA!

The main requirement is the ability to receive SMS and this is possible through 3 means:

1. Short codes
2. Virtual Mobile Numbers
3. GSM Modems or mobile phones which can connect to your computer and interact via AT commands.

VMN's are not available in the US - so you can rule this out.

If you go for #3, you will need to invest in the equipment and learn about how the whole AT thing works. You then program your MO messages to be forwarded to your server. There are companies that can host sim cards for you, but the American ones charge extortionate rates and, as they won't allow you to send the response through the SIM card, you won't be able to send any response.

If you go for #1, you would need to find an sms gateway that provides north american short codes. Short codes are very expensive, but you can lease a keyword at a fraction of the cost.

When you go global, then you would need to use virtual mobile numbers (because they are internationally accessible), but until then, your best bet would be #1.