How to get a subpoena to obtain personal cell phone records so that I can show documentation of contact when I sponsor my wife?

Typically in the United States the only way a subpoena is issued is when there is a lawsuit filed and the subpoena power is utilized to have a third party produce documentation or attend a deposition when such would normally not happen by the mere asking for the document production or attendance for questioning.

If you want your personal cell phone records of calls that you placed, I would simply contact your telephone provider and ask its representative to provide you with copies of your monthly cell phone records (bills) going back so many months.