Sending SMS from web app

Synopsis

Client’s website already has a full user management system up and working. This blows me away but they are actually dealing with potential user’s that do not have email accounts but no shortage of cell phones. They want user’s to be able to sign up with a phone number and get text messages that regular user’s receive. User’s are brought into the system almost exclusively through a in person meeting.

The Question

I’m not really looking for any ideas on implementation code wise. I built the whole thing and can fairly easily get it up and running. My question revolves around potentially getting blocked by the user’s cell phone provider.

They aren’t really busy enough to get flagged at this point. I just can’t imagine that cell phone providers don’t have some sort of limit on how many texts you can send from your IP address before its blocked without paying some type of fee or minimally getting on a whitelist.

I know there are services that provide API’s to accomplish this but the client is convinced a third party service is not needed.

Anyone dealt with anything like this?

Sidenote

Yes I’ve discussed the potential problems they can run into. They really want it and I’ve received the “Duly Noted” response. So its kinda a good deal for me, I can try something new without putting it all on the line.

This sounds like an interesting idea, I’ve actually been looking for something very similar myself, but unfortunately, I’ve not had the success of actually getting this to work!

Can anybody point me in the right direction?