how can I un-block phone number

I'd be more than happy to give you this info!

Access Settings > Phone > Blocked Contacts > Edit in the top right, then remove the number from the Block List :) Hopefully that solved your issue!

Settings>Phone>Block calls... choose a number, slide on it to the left, then un-block it.

Regards

Giulio

See: How to Unblock a Number on iPhone or iPad - Lifewire