First Contact Resolution is a concept not a measure
First Call Resolution is a good concept not a good measure.
So if I phone in to report a technology problem with my broadband, the company may need to send an engineer to my house. So if you try to measure First Contact Resolution on the call - was my problem resolved? No. Three days later the engineer calls and fixes my problem. Now has it been resolved. The only way to measure this is to send a survey, which now my broadband is working again, I may not complete.
What you can measure better though is the number of repeat calls.
But there may be many reasons why a caller may need to call back, for instance to get extra information that may need to be provided.
The measure that you can influence on First Contact Resolution is the number or percentage of repeat callers. If you are good at First Contact Resolution then the Repeat Caller Rate will fall.