Section 12(8)(a) of the IGST Act, 2017 is applicable to the situation explained by you. Hence the place of supply is Kolkata, the location of the registered person who has received the taxable supply. The status of transporter is unregistered so liability of GST is cast upon you as registered person receiving the supply.. You as a registered dealer are receiving the transport service by road from an unregistered transporter whether from Mumbai or Telangana it makes no difference as place of supply is Kolkata as a registered person receiving supply of service.
SECTION 12. Place of supply of services where location of supplier and recipient is in India.
(8) The place of supply of services by way of transportation of goods, including by mail or courier to, -
(a) a registered person, shall be the location of such person;
(b) a person other than a registered person, shall be the location at which such goods are handed over for their transportation.