Tigo Sri Lanka.

Tigo trually a global band is launched by celltel to provide GSM network to cover all corners of the island, and successful introduction of E PIN/RELOAD making prepay services more affordable, and the introduction of GPRS/EDGE services. Celltel’s existing customers can continue enjoying current services, and can use Tigo cards for reloading and/or can change to the attractive new Tigo tariff packages, by dialing a short code.

You can send SMS using mobile phones through Tigo service. There is no web based SMs sending facility available at the Tigo website. Visit Tigo's value added Service click here Tigo VAS Tigo

As per Celltel website only members who join the website cann send SMS from the web based SMS service Mobile Messaging Will Never Be The Same Again.
Send SMS using Celltel website click here Celltel