ASAP smartphone app "imsy messenger" why need phone number to register?

download an "imsy messenger" app when i first run it they ask for phone number and name to register! do anyone use this? if so tell me about why do they need user phone number to register? it is safe to give them the phone number? would they ask for money since they have ppl phone number? it stated it FREE! what is the catch too this kind of app give phone number to register and use it?