[Buying and Selling by Telephone]

The answer—and Allah grants success: If the contract of sale is concluded by telephone, it is valid so long as the voice is recognized and there is no mutual denial. If, in such a sale, a deposit (ʿarbūn) is taken, then the deposit is part of the price; the purchaser is obliged to pay the remainder of the price and then receive the item sold.
Moreover, they have permitted the validity of contracts by written correspondence and letters; therefore, the telephone as a medium is even more deserving of validity.
Yes, it is the custom of merchants to transact by telephone while the seller, at the time of concluding the sale, does not own what is the subject of the contract; once the contract is concluded, the seller goes and purchases what was agreed upon. This sale is invalid and one should not enter into it, due to what has come therein of prohibition from the Prophet—May Allah bless him and his family and grant them peace.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2