بسم الله الرحمن الرحيم
The nikah is not valid by telephone or video calling or online voice calling when the boy and girl are in two different places, as they are not present in a single meeting in all the three cases. However, nikah may be done through any wakil. i.e. the girl appoints someone as wakil for her nikah and wakil may be a relative as well and then that wakil may solemnize the nikah of the girl in the presence of two witnesses as per the Shariah ijab-o-qubul (i.e. proper exchange of nikah words) then nikah shall be valid.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.