There is no specific available tip for this expression.
A good HTML content optimization can not be restricted to only one expression. The whole theme around the expression is to optimize. The following tagcloud provides you main expressions taken from your competitors and missing in the audited web page. Click on the keywords close to "appeler etranger moins cher" to get more tips.
To go deeply :
You can optimize deeply with the following keywords. Choose them taking in account these 2 rules :
Choose the keywords close to your keyword "appeler etranger moins cher".
The redder a keyword is, the more significant it is for your SEO.
