Mont. Code Ann. § 72-38-217

If a trustee or beneficiary has served and filed either a notice of appearance, in person or by counsel, directed to the petitioner or the petitioner's counsel in connection with a particular petition and proceeding or a written request for a copy of the petition and has given an address to which notice or a copy of the petition may be mailed or delivered, the petitioner shall mail a copy of the petition to the trustee or beneficiary within 5 days after service of the notice of appearance or receipt of the request.