If you are talking about page score in Adwords.
To get a good page score context is more important than KW density.
Though stuff like loading speed, and over time bounce rate and time on page all count. Keywords work only well in conjunction with context of the surrounding content.
So "Cell Phone" bid.
500 words with Cell phone at 5% MAX (this is Adwords score not to rank in natural SERPS - the same rules DO NOT apply)
Then make sure stuff like "4g", "IPhone", "Mobile network", "Data allowance" and all other context making words you can think of are in the mix.
After a few hundred referrals, the page score can change depending on time on site, bounce rate, actions on site etc.
Scritty