Your landline company controls their network. You could not program your landline phone to block unknown callers; you had to ask your landline provider to. Likewise, your iPhone does not control your cellular providers network. So you have to ask your cellular provider to block calls, or at least ask if the can or will. Your phone actually cannot block any calls; all it can do is send them to voicemail, which is not a true block.
Solicitors have nothing to do with it. Today solicitors do not call with caller ID blocked; instead they use a false caller ID. Most calls with blocked caller IDs are legitimate calls; for example, your doctor returning your call from a personal phone whose number is unlisted. The phone spammers use faked caller IDs that look like a phone in your area code and exchange (NPA-NXX for the aficionados).