The response of Ayatullah Mahdi Hadavi Tehrani is as follows:
The only way for a deaf person to divorce his wife is through writing and of course, he must have two witnesses when he does so.
If he is unable to write, he must make the divorce understood through gestures that clearly do so, in a way that everyone understands that what he is doing is divorcing his wife.
Also, his expressing the divorce must be done in the presence of witnesses, therefore it must be done live and in person or in a way similar to that, like through a live video conference where there are witnesses. In this case the divorce will be effective.
For further information, see:
1- Divorcing via text messaging (SMS), Question 376 (website: 387).
2- Divorcing via video conference, Question 7766 (website: ??????).