Nowadays when a guy and girl in different countries get engaged, it is extremely difficult to abstain from contact with each other, especially when the nikah is put to hold for a while. In such cases, it will be permissible to conduct their nikah over the phone if the procedure is done properly. This question was answered earlier this year:
Respected Brother in Islam,
Firstly, it is important to mention that there is no concept of engagement in Shari’ah. If the parties of the girl and boy agree that marriage should take place between them, they will still be non-mehram to each other until the actual nikah takes place. Any form of contact between them will not be permissible until then.
That being said, many times when both parties agree to have a marriage in the future, it is difficult for the girl and boy to control their desires and hold off all contact until the nikah, especially in cases when the nikah is delayed for a long period of time.
Shari’ah encourages having the nikah done as soon as possible for this very reason so that these types of instances may be avoided. If for some reason the nikah cannot take place until a later period, the boy and girl should then fast in order to control their nafs.
In principle, nikah conducted over the telephone is not permissible since it does not fulfill the criteria needed for acceptance. In order for nikah to take place, both ijaab (proposal) and qabool (acceptance) must take place in the presence of at least two witnesses. Even with the presence of witnesses, they will not be there to physically witness both the proposal and acceptance.
However, as an alternative, it will be permissible for you to appoint someone living in the country of the girl as an agent on your behalf to contract the marriage and to pronounce the proposal and acceptance on your behalf. For example, if you are person ‘a’, the girl is person ‘b’, and your appointed agent is person ‘c’, then ‘c’ will initiate the offer to ‘b’ on behalf of ‘a’. Person ‘c’ can address ‘b’ in words such as:
“Being an authorized agent of ‘a’, I marry ‘a’, son of so-and-so to you.”
Thereafter, ‘b’ will reply:
“I accepted this marriage.”
This process must take place in the presence of at least two Muslim, sane, mature male witnesses or one male and two female witnesses. This will be considered as a valid marriage contract and the boy and girl will be husband and wife according to Shari’ah.
(Al-Hidayah, 2/306, Maktabah Shirkah)