I had a similar problem a couple of months ago when I was trying to figure out how to receive and send international texts.
Sprint is simply hopeless when it comes to international texts to and from the UK (the O2 service in the UK, shown as compatible with Sprint on Sprint's own site). Sometimes they go through immediately. Sometimes they take days. Sometimes they never arrive at all.
I tried third-party sites as you did. Only one of them worked at all -- I can't remember which -- and that took hours.
I never had any problem with texts in my old T-Mobile days. Perhaps T-Mobile will take over Sprint -- as recently rumored -- and fix it ....
