Is it permissible to renew a marriage contract over the phone in the presence of two witnesses? And is it valid for the wife's guardian to merely say, "I have given you my daughter in marriage under the same previous contract," while I say, "I have accepted for myself"? If that is not valid, what are the conditions for renewal, given that the spouses are in two different countries?

A clear divorce issued by the husband to his wife is effective, even if the husband is not joking. If you divorce your wife before consummation, and there was a valid private seclusion (khalwa (definition) shar'iyya), you may take her back before the 'idda (waiting period) expires. However, if there was no valid private seclusion, then the wife becomes irrevocably divorced from you and is not permissible for you except with a new marriage contract. Renewing the contract over the phone is not valid, but it is permissible for you to appoint someone to accept the marriage on your behalf in the presence of the contract.