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