37 C.F.R. § 11.307 – Practitioner as witness.

(a) A practitioner shall not act as advocate at a proceeding before a tribunal in which the practitioner is likely to be a necessary witness unless:
1. (1) The testimony relates to an uncontested issue;
2. (2) The testimony relates to the nature and value of legal services rendered in the case; or
3. (3) Disqualification of the practitioner would work substantial hardship on the client.
(b) A practitioner may act as advocate in a proceeding before a tribunal in which another practitioner in the practitioner's firm is likely to be called as a witness unless precluded from doing so by §§ 11.107 or 11.109.