how to write code in table trigger to send SMS upon any modification?

There are two parts to it.

1. Create a trigger after INSERT/UPDATE/DELETE. Triggers can work for both sql and object access:

Trigger NewTrigger1 [ Event = INSERT/UPDATE/DELETE, Foreach = row/object, Language = objectscript, Time = AFTER ]
{
    set ^dbg={fieldname*N}
}

2. In the trigger code send an SMS. You can either use an API (a lot of them available) or talk to a GSM modem.