I'm totally new to co-reg, and joined Silverpath few days ago.
Let's suppose i come out with a nice angle in order to approach new users; do you think i can be profitable collecting only their phone numbers ?
Split test. 
Yea, you can... but RPU is usually better the more info you collect.
My opinion is that when we have only the cell phone field, the user might start thinking if we'll start spamming them with text messages, calls, etc.
So maybe we should play it down and ask for other info as well - the point is to engage them deep enough..so that when the offer on the path asks them to 'confirm their cell number' they should have put in too much to quit at that point.