Very useful #TestNumbers (for #UK #IE #USA #CA and #MX ) for testing all kinds of #telephones (from your #mobilephone to #VOIP and even any remaining #landlines) - they provide the same level of tests that #Openreach and other telcos have access to, you can check if #DTMF is working well and that #codecs or poor #network conditions aren't mangling the audio..

https://thetestcall.blogspot.com/