I phone a U.K. number and get the unobtainable tone. The phone allows me to use my mobile network from the same menu and the call connects . This is happening on two numbers so far. This could be the handset but it works for most numbers.
I would try another landline but I am schielding. I would suggest you dial one of the numbers and see if you get it ringing. It is a business to you could just hang up if it rings. If you agree I can send you a number