Financially, dial-a-porn is good to telephone companies, which keep about a third or half of the revenue the services generate. The privatization of British Telecom and breakup of AT and T in USA have generated premium rate services. Nothing in law restricts what two individuals can say to each other over the telephone. So unless the service provider can be judged somehow guilty of "inciting debauchery", the only way to prevent pornographic telephone services is to close down the entire service category - as France did with chat lines that turned out to be pornographic.
In France "pink messages" consist of porno messages designed to put certain Minitel (videotex network) users in touch with each other, or of advertisements for sexually explicit products or services.