iPhone randomly blocks numbers

Hello, SaraVD.

Thank you for using Apple Support Communities! I understand that you're noticing calls from numbers not saved in your contacts list are being blocked.

Are the numbers appearing in your list of Blocked Contacts in either Settings > Phone or Settings > Messages?

If the numbers don't appear in those lists, but are going straight to Voicemail, check to see if you have Silence Unknown callers enabled:

Detect and block spam phone calls - Apple Support

I hope this helps! Take care.

See if you can turn the setting SILENCE UNKNOWN CALLERS shown in the picture to OFF.