I had a similar problem with my landline which was mainly my doctors phoning on their consulting room phones, (not the main surgery number), which were blocked for return calls. I tried to sort it out with BT, but they never solved the problem. In the end all doctor’s phone appointments go to my mobile, where there is no problem. I did have blocks on my landline phone to stop junk calls, which may have caused the problem.
Are they only one type of call that go direct to voicemail? If so, you may have inadvertently blocked that type of call.