[The testimony to acknowledgment by telephone]

The answer – and Allah is the One who grants success – is that if a person hears the voice, and ascertains it, and ascertains who is speaking and knows him, then there is no objection to testifying and to the validity of that testimony.
The scholars of the madhhab have permitted the testimony of a blind man in that which does not require sight and direct observation, and on this basis it follows, in the madhhab, that testimony is valid regarding an acknowledgment heard by telephone and the like.

[Testimony regarding transactions by telephone, by analogy with the blind person]
Speech by telephone is like the speech heard by a blind person. Whatever the testimony of a blind person is accepted in, the testimony of the one listening by telephone is accepted in; and whatever it is not, then not. This is in the chapter of testimony to a voice heard by telephone.
It is valid to conclude agreements by telephone, if the witnesses know the voice and ascertain it – whether it be an agreement of sale or otherwise, such as a contract of muḍārabah (profit-sharing) or partnership – except in what requires hand-to-hand exchange (taqābuḍ fī al-majlis), such as selling one kind for its like among measured commodities, and currency exchange (ṣarf) and forward-sale (salam).
Yes, ṣarf and salam and selling one kind for its like may be permissible by telephone, on the condition that each of the buyer and seller has a balance and an account in the bank, and that each of them transfers into the account of the other what is due from him.

Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.2