how to make either one of the field mandatory based on user input.

[kkraju]

how to make either one of the field mandatory based on user input, for example mobile phone or email, if user inputs phone, the email should become optional field and vice versa.

[Uncle Rob]

UI Policy should do the trick:   Creating a UI Policy - ServiceNow Wiki

If field 1 is empty, make field 2 mandatory.   Reverse if false.

If field 2 is empty, make field 1 mandatory.   Reverse if false.

[fletchermichael]

I had a similar question, and your solution worked, thanks Robert.