Analyze Only the last 7 digit value eg"telephone number"

I have a mappings like this
{
"mappings": {

"telephones": {
  
	"properties": {
		"test": {
			"type": "string",
			"index": "no",
			"include_in_all": false
		},
		"rawTelephone": {
			"type": "string",
			"index": "no",
			"include_in_all": false
		},
		"telephone": {
			"type": "string",
			"copy_to": ["phonekey"],
			"index": "not_analyzed",
			"include_in_all": false
		},
		"type": {
			"type": "string",
			"index": "not_analyzed",
			"include_in_all": false
		},
		"phonekey": {
			"type": "string"
}

}
}
}
}

Now the values of telephone is copied to "phone key" from the results of phone key the last 7 digit should only analyzed
how to analyze it?