A method is disclosed that avoids the need to enlarge the translation tables held by Signal Transfer Points (STPs), for the purpose of effecting global title routing of certain signalling messages to enable number portability. Signalling links are monitored to detect messages needing to be routed by global title; the global title of each such message is then checked to see if it relates to a ported number and if this is the case, the global title is modified to a number that will result in the appropriate routing of the message on the basis of the existing translation tables of the STPs.