A method for ascertaining a billing tariff for billing for a data transfer, where a mobile communication terminal which is involved in the data transfer and has access rights for a data mobile radio network is situated in a visitor mobile radio network. The method involves setup of a data tunnel, used for transferring the data, prompting transfer of an item of location information which relates to the communication terminal from a switching center, temporarily associated with the communication terminal, in the visitor mobile radio network to the network gateway node in the data mobile radio network. The network gateway node transfers the item of location information to a billing system, and the billing system uses the item of location information to establish that the communication terminal is situated in the visitor mobile radio network. Thereupon and the billing system ascertains a billing tariff which is associated with the visitor mobile radio network.