good morning is translated with shubh prabhat . but there is no custom in Hindi to wish good morning instead we start with namaste. which means i bow to you. so if this is not for translation start with namste instead of shubh prabhat.
how can i help you? is translated as main apki kya madad kar sakta hun? (literally it is I yours what help do can is?) remember it is not how it is what.
who is calling? this is in response to phone call usually so in that case you say
kaun baat kar rahe hain? if the person calling is male and kaun baat kar rahi hain? in case of a lady calling. remember that n in hain and hun are nasalized like french un .