The Quran provides guidance for all situations, but we need to look at other issues to see how they can be applied to the problem at hand.
I have my reservations regarding phone witnessing, and this includes all types of communication where a physical witness is not present [video conferencing etc], based on the following verse:
2:282 O You who have chosen to be graced with belief! When you transact a loan for any period, you shall write it down. An impartial scribe shall do the writing. No scribe shall decline to perform this duty, and write as God has taught him. The debtor, or in case he or she is incompetent, his representative shall dictate the document honestly and be mindful of his Lord, and diminish not the amount. There shall be two witnesses to the transaction from among your men. ......... Do not tire of writing the details, no matter how long, including the time of repayment. This is equitable in the Sight of God, assures better witnessing, and eliminates any doubts you may have. When it is actual merchandise that you transfer among yourselves from hand to hand, there is nothing wrong for you if you write it not, but have it witnessed. The scribe and the witnesses must be held harmless for their services. If you harm them, it would be wickedness on your part. If you follow God's Commands He will increase you in knowledge. These are the Injunctions of God Who is the Knower of all things; intentions, events and actions.
When we do monetary transactions we need two witnesses to sign that they agree with what has been undertaken. How much more important is the witnessing of a marriage contract and divorce, for should something go wrong, peoples’ reputations are at stake.
I have heard of cases where a man and woman have married over the phone, each one having witnesses at each end to hear the agreement, but they were still required to sign documents there and then to prove what took place.
I doubt whether the testimony of a witness who used a phone would be valid in courts today; it would be objected to by the judge as not an ‘eyewitness’.
We sometimes make things harder for ourselves than is necessary; extremes should be avoided, this next quote is about wealth, but can equally apply to avoiding any extreme situation;-
25:67 And those who, when they spend their wealth, are neither wasteful, nor stingy. And (they know that) there is always middle ground between the two extremes.
Try to do things in a balanced way:-
55:9 Therefore, establish Balance in the society in absolute justice. And never belittle the Scale of Justice in the community and in all your transactions with your own ‘self’ and with others.
If a man is separate from his wife and they divorce, each party needs to appoint witnesses to prove the signing of divorce papers, or the woman can divorce the man if he has deserted her and is not available; the couple need not be together, but witnesses need to be present to verify everything.
Any witness will do, there are enough human beings on this planet so finding two should be no problem!