Since updating to 16.3.1 I am unable to call 800 numbers

Many users who have updated to iOS 16 and above must have a similar issue if the issues stated by you are due to the iOS.

Unless you state what the issue is many in this forum may not be able to respond positively to the issue you are facing. 

With the limited information that you have provided, It is hard to even guess the problems faced by you. 

You may get faster and better responses to your query by --> Writing an effective Apple Support Commun… - Apple ...

Please re-post with detail so that someone else can help.

Making a voice call is a Carrier Specific feature. Contact your Carrier, please. There are no settings on an iPhone which will bar certain numbers with certain prefixes. Only Carriers can do this.