A method of signalling in an AIN, in which in an SSP, the initial message of a new transaction is addressed to a Service Global Title (SGT). The SSP translates that SGT to a first address of the first choice SCP. The SCP responds with an acknowledgement message which has a second address of the SCP as its source address, and includes the Nodal Global Title (NGT) of that SCP. The SSP sends subsequent messages for that transaction addressed to that NGT, and the SSP translates that NGT to the second address of the SCP. When the SCP is taken out of service, the first address is made unavailable, so the SSP sends new initial messages to the second choice SCP, but subsequent messages for existing transactions may still be sent to the second address of the first choice SCP. When all existing transactions have ceased, the SCP is taken out of service.