In addition, try asking relatives, friends etc who are likely to call you, whether they have noticed any problems.
If they have not, it makes it more likely that the calls are of the variety that others have described.
I was amazed about 25 years back before this automated nuisance calls era, that for a period approaching a year, our phone would very frequently ring, then stop before we reached the solitary hand-set of that era.
However on one occasion when I happened to be beside it, I managed to answer quickly, to discover a relative 450 miles away, in conversation with our local police station, about 1 mile away.
On making enquiries with various relatives, I discovered that several had encountered this, although the numbers were distinctly different, thus most unlikely to be simple mis-keying, of the "one digit out" nature.
One relative had been threatened with a charge of making nuisance calls etc!
When I reported it to GPOT/BT, the local Exchange Engineer called me to explain he had found that somehow one of the Cross-Bar units involved (successor to Strowgers) was making simultaneous connection to the police station line - where with staff quickly answering, explained the ringing ceasing quickly in our house.
During that period, we had no problems making calls, ie "ringing out".
BUT I was very surprised that none of the relatives had mentioned the problems to us, given the extended period that they occurred over.
Another frequent problem but at work, back then was that from 1972, we had dial-up facilities, to make contact with a main-frame in Birmingham, about 330 miles away, so again a well-known number to myself and my colleagues.
All too frequently, we would be connected regularly to either of two other numbers.
With the first, as we did not quickly hear the remote computer response, the warbling "handshakes" of dial-up modems, we would hang up and try again.
It was an "Ex-Directory" number in Birmingham, where the lady of the house had noted the phone bell ringing; but had generally been in her bath, around our starting time of 8 am, so had not answered.
GPOT put it down to the two-bare-wires line of the period, passing through the branches of a tree in her garden, so she was rather re-assured one morning, when I had waited, she had answered; and we were able to establish that it was our calls being mis-routed.
The other wrong number was usually answered very quickly, as it was "Greater Manchester Police HQ".
Basically given the technology of the era, there was very little to nothing that we or the GPOT could do about it.
I could also tell if there were storms in the Lake District by the qualities of the calls.