[On Contracting by Telephone]

Answer—and Allah is the One who grants success: It is valid if the witnesses are present, hear both the offer and the acceptance, and know the voice and its owner with certain knowledge.
This is based on what the scholars of the madhhab stated: the testimony of a blind person is accepted if he recognizes the voice and that recognition yields certainty—then his testimony is accepted.
Source: Min Thimār al-ʿIlm wa al-Ḥikmah vol.1