Are ny sentences correct?

They are all correct (if you put 'the' in front of 'phone' in #3), but they all sound very formal. Most native speakers would use 'who' rather then 'whom'.

Thanks for your answer. Shall I write as below?

1. Who should I ask?
2. Who should I contact?
3. Who are you talking to on phone?

Also, how can I write same sentences in past tense?

Chandrasekhar Reddy Who are you talking to on phone?

NO. I said you needed the definite article before 'phone'.

1. Who should I ask?
2. Who should I contact?
3. Who are you talking to on the phone?

Would you please guide me to write above sentences in past tense?

Chandrasekhar ReddyAlso, how can I write same sentences in past tense?

1. Who(m) should I have asked?
2. Who(m) should I have contacted?
3. Who were you talking to on the phone?

Thank you very much.

Chandrasekhar Reddy1. Who should I ask?2. Who should I contact?3. Who are you talking to on the phone? Would you please guide me to write above sentences in past tense?

There is no past-tense version as such of the first two. If you want to refer to a past-time unfulfilled obligation, use 'should have'.

Who should I have asked?

Got it. Thank you so much.