Yes, you can call any US number (mobile or landline) with Flyp.
While calls are placed and received over your cellular provider’s network, you will not be charged international rates. Phone calls that originate from the country in which you purchased your mobile plan will be considered local calls. That means if you have an unlimited calling plan with your carrier, Flyp calls a free! If you do not have an unlimited plan, Flyp calls will cost as much as calls to a local number.
Please be aware that Flyp is not a roaming solution and that international rates may apply if you are outside of your home country.