Unless they are making persistent nuisance calls, you can't stop them. It may be a sad comment on their management capabilities, but the phone is there for contacting people so if they want to ring you they can, same as eveyone else
As a general rule, people who work in the public sector are protected and should not receive calls from work. There is no law against it in the private sector, but if you are called constantly, that could be seen as harassment.