the reason for not having clocks is simply to induce the players to forget time and not realise how late it is getting or how long they have spendt there. The longer the player stays the more money the casino makes.
The possibility of using phones to cheat is one reason cell phones are banned at the tables. Another is that waiting for a player to take a call slows down the game. Casinos make their money through volume. Fewer hands per hour means less money wagered and smaller profits for the house.