DUBAI, April 21 (Reuters) - Saudi Telecom Co (STC), the Gulf's **.1 Telecom operator by market value, missed analyst estimates despite reporting a 54 percent jump in first-quarter profit on Monday. The firm, which competes domestically with Etihad Etisalat (Mobily) and Zain Saudi, made a net profit of 2.39 billion riyals ($637.3 million) in the three months to March 31, up from 1.55 billion riyals in the prior-year period, it said in a bourse statement. ...