my question is about a nikkah over the phone...

One of the conditions for the validity of Nikah is the witnessing of Nikah.For that the physical presence of the witnesses is necessary. A nikah conducted by telephone does not fulfill this criteria as both parties are not in the same place and it is not possible to fulfill the conditions of the physical presence of the two witnesses in one place.
As an alternative, proxy of Nikah by telephone may be conducted like a written proxy for the Nikah.If the future spouses are for example, in two different countries, the representative of the girl may telephone the boy and grant him permission to conduct the Nikah.There should be two witnesses on the side of the girl who also verbally testify being present and being witnesses to the permission being granted to the boy to marry.
If the boy is certain of the permission being granted to him and there is no doubt whatsoever in the proxy by telephone, the boy may now, independently perform the Nikah relying on the proxy by telephone.
In brief, a Nikah through telephone cannot be conducted, proxy of Nikah may be conducted trough telephone.

And Allah Ta’ala knows Best.
Mufti Ebrahim Desai