A separate exemption certificate (as required by §§ 49.4253-3 and 49.4253-4) shall be furnished for each message paid for as a separate item, but where periodic payments are made, a blanket certificate (for a period not to exceed four calendar quarters) may be accepted as evidence of the right to exemption. An agent of a telegraph, telephone, radio, or cable company should not accept an exemption certificate unless satisfied, on the basis of proper credentials or otherwise, that the person who signed it is the person whom he represents himself to be and that the exemption claimed is allowable under the law. Exemption certificates should be retained with the record of the services rendered for inspection by internal revenue officers as provided in section 6001 and the regulations in Subpart G of this part.