Bismihi ta'ala

In principle it is permissible to conduct a nikah ceremony over the phone, as long as all necessary requirements are met. Among these requirements are:

1. Consent given by the bride's guardian. 

2. From a social and akhlaqi perspective, the groom has also obtained the blessings of his family.  

3. Mahr having been agreed upon by both parties. 

4. It is highly recommended for there to be witnesses who are witnessing the ceremony, and can also sign any documentation the person conducting the nikah might issue. 

5. Anything else that would be required legally, or Islamically. 

6. I would also recommend that the bride and groom learn their lines, and read about the process for the Islamic marriage contract, so they are aware of the procedure. 

And Allah knows best.