37 CFR §11.307

1. (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.
2. (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.