Port out to a discount voip service, get a new mobile number, forward the old one to a recorded message that explains the situation then forwards the calls on to your new number if callers press 1.
Should cost a buck a month plus a penny a minute for the forwarded calls if you use a service like voip.ms or other discount voip, which for the purpose of screening inbound calls will be adequate.
Only downside is you will probably lose your current mobile plan when you port out, which if you are on contract might be pricey.