As the speeds are well below the estimates then it is worthwhile checking for a fault and log one if necessary.
But test the phone first and then if necessary the broadband. Links below.
First thing is to check for a phone fault (if Plusnet provide the phone start here https://www.plus.net/help/phone/phone-line-troubleshooting/ ) If you do have a phone voice fault, report it as such and do not mention any broadband issues. And then when that is fixed, see if the broadband is better.