I use the test um lil buttie, it has dsl indication and a screen with onboard dtmf decoding, cli, line voltage, line current and polarity indication. tbh though I dont use pstn for intruder alarm signalling but it is still very prevalant in warden call and medical alarm signalling, and the dtmf decode has been very handy on a number of occasions. I bought a few when comtec were having a clearout at around £13+vat