Question:

What is the Shar’ī ruling regarding Ṭalāq over the phone? When does it take place and is it necessary to have witnesses?

Answer:

Ṭalāq issued over the phone is legally valid. There is no need for witnesses and it takes place as soon as it is issued.

Fatawa Darul Ulūm Zakariyyā, Vol. 4, Pg. 52

And Allāh alone knows best