You can set your iPhone to ignore callers not in your contacts.
We still have an old landline phone number, but we use a VOIP service, Ooma. Their software gives us a range of anti-spam call options and we have it set up to permit calls on our Contact list to ring through, to ring continuously for known spammers and to send to voicemail for all other calls. Software also notifies us and sends an audio file to email for all voicemail. Requires a bit of fine tuning and keeping your Contact list current, but works well.
We still have an old landline phone number, but we use a VOIP service, Ooma. Their software gives us a range of anti-spam call options and we have it set up to permit calls on our Contact list to ring through, to ring continuously for known spammers and to send to voicemail for all other calls. Software also notifies us and sends an audio file to email for all voicemail. Requires a bit of fine tuning and keeping your Contact list current, but works well.