To what extent is a marriage valid that was conducted over the phone, with the woman's son acting as her guardian, given that her brother was preventing her marriage, and knowing that the parties to the contract were in different countries and there was a delay in finding the second witness?

The woman's most rightful guardian for marriage is her father, then her grandfather, then her son, then her brother. If the marriage contract is concluded during the 'iddah (waiting period), it is permissible according to the Shafi'is and some Hanbalis. Her guardian can directly conclude the marriage or appoint someone else as an agent, and this agency can be done over the phone.