Can't register by phone

For the last 2 weeks now, whenever we try to sign someone up to our twitter account using text message (not via the web), they never receive their confirmation message. I have been signing them up myself texting 40404 from their cell phone with the message: follow viatrading (or follow viatradingespan as we have 2 pages). People are NOT being added to our list doing it like this!
For the last 2 weeks we've ONLY been able to get new followers via the web.
Please help! Twitter is very useful for our non computer literate customers but as of now, we cannot sign them up. Thank you!