I did a lot of tests on free SMS texting sites prior to setting up my own site
here .
Most of them seemed to be useless.., very unreliable. However, when I found CBFSMS they were completely different. Their service occasionally gets overwhelmed by weight of numbers. However, when that is the case you know about it right away. My experience of it, in lots of tests, is that my messages were received as quickly as paid for text services.
The popups? Something has to subsidise a free service.
If you're interested..... Long ago in internet terms, 6 years, there was a guy whose name I can't remember, who believed that no one should have to pay money to communicate.
He latched on to a little known fact, that telephone networks had spare 'channels', I think they were used by engineers to test the networks. The guy set up a site
MTNSMS to give worldwide free SMS texting. It became hugely popular, for a few years. It was a great site, and also looked very good aesthetically.
Eventually the global corps decided that enough was enough of people using their networks for free, and started charging. Which was more or less the end for MTN.
The site closed, and eventually the domain lapsed. The domain got bought up by what became CBFSMS who realised that to survive they had to have a business model which included bringing some money in. Hence the popups.
If you're interested and type in MTNSMS in Google websearch or image search, you'll find traces of what MTNSMS once looked like.
Andrew