拨 means dial(verb) , 通 is the complement of result(结果补语)。 The complement which tells the result of an action is known as the complement of result. Aa a rule , it is a verb or an adjective that acts as the complement of result. For example .打通， 写对 ， 买到 ，听见，写错，打哭. (通对到见错哭 are all the complement of result.)

dial and connect

表示输入号码后等待对方接通

表示电话正在响铃中。