awk 'FNR==NR{tarif[$1]=$2;next}{cust[$1] += ($2 * tarif[$3])} END{ for (i in cust) print i,cust[i]}' file2.txt file1.txt
666666661 53.98
666666662 56.1
666666663 10.71
666666664 44.46
666666665 131.04

awk '

# process first file (file2.txt) to build up the lookup table
NR==FNR {
	tarif[$1]=$2
	next
}

# process all the records in the second file (file1.txt)
{
	cust[$1] += $2 * tarif[$3]
}

# output the total per phone number 
END{
	for ( i in cust ) {
		print i, cust[i]
	}
}' file2.txt file1.txt