Thank you for your question.
The short answer to your question is 'yes'. A deposition may be taken through any method that all parties and witnesses agree upon. If this makes sense and the attorneys are alright with it, along with the witnesses, there is no reason that a deposition cannot go forward via telephone.
However, if a subject is in another state, you may not have the power to compel them to attend a deposition if they do not wish to be deposed. A regular deposition subpoena will not work on a non-party, so you may need to go into court for a special order to obtain jurisdiction over someone in another state if they do not wish to be involved in the case.
The other issue is, if the case is going to trial, is deposition via speaker phone sufficient? As an attorney, depending on how important the witness was, I may be concerned about not being able to see the witnesses mannerisms. This however is only of paramount importance if they are a key witness in my case.
Then you have the issue of having a court report present. I would assume that the court reporter would need to be present in the room with the witness. The deposition transcript would then need to be handled according to the rules of the state wherein the witness was being deposed.
Please let me know if you have any other questions. Otherwise please hit "ACCEPT", so I may receive credit for my response. Tips and feedback are appreciated.