The short answers to your questions are (1) Yes, and (2) No.
(1) Yes, you can subpoena the alleged paramour to testify as a witness at a deposition and/or at trial. However, a subpoena does you no good unless you can validly serve the subpoena on the person named in the subpoena. So, you need to find out where she might be (either her home or possibly her work address). A private investigator can usually find out information like this in fairly short order. However, I would suggest that you start by requesting that your husband provide the woman's address in his Answers to Interrogatories.
While he will probably initially file an objection to the question, it is unlikely that his objection will stand. (This is provided that her address is not the same as admitting to the adultery, but is reasonably calculated to lead to evidence of marital fault that is relevant to the divorce case).
(2) Since the divorce case is between you and your husband, the paramour is not a party to the suit. While the court will issue subpoena duces tecum (subpoena of documents) to third-parties if the third party has custody of discoverable records, generally those requests are limited to records of the parties.
For example, you might issue a SDT to your husband's bank, his employer, his 401(k) plan administrator, etc. to provide your husband's records. However, it would be far more daunting to overcome the objection of a third-party to a STD of that third-party's records (i,e., another account holder at the bank, another employee at the company, another person's phone records). You would have to have a really compelling reason for the court to violate a person's right to privacy in favor of a litigant, especially when that person whose privacy was being violated was not a party to the litigation.
There is a much easier way in which you can obtain this same information without violating a non-litigant's privacy -- you can request your husband's phone records in both a request for production to be served on him and in a SDT to be served on your husband's phone service provider(s). The only calls on the paramour's phone records to which you have any legitimate interest in this litigation are calls made to or from your husband, which should also be on your husband's phone records.
This response does not create an attorney-client relationship and is intended for general information purposes only.
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