create a group on your pc, add phone numbers then download to phone, then send from your phone, all free if you have unlimited texts.
I used to do exactly this when I was bussing kids to raves. Then I got hit by O2 for sending over 1500 simultaneously from a non-business account.
Apart from Bebo spamming this was the most efficient way of advertising to them. Then Facebook took over and it was the only way to do it.
We use texts with all of our floor maintenance customers with amazing success. Many are busy professionals without time to answer a call or email due to meetings or being out of the country.
A quick "Hi Jim, How's the floor looking? It's 6 months since the last visit. do you require another?"
"Yes" he's too busy to say please

Previously we would have made several attempts to get in touch with him.