Absolutely you can.
With Dingtone, you’re allowed to call any number in over 200 countries around the world, landline or mobile. Naturally, as a Dingtone user, you’re allowed to call your friends with Dingtone app installed.
The only difference between calling to a Dingtone friend or a regular number lies in the payment. If you call a Dingtone user, NO charge is required. If you call a regular number, Dingtone credits will be consumed. Therefore, it’s a good idea to invite more friends to use Dingtone so that free calls can be expected, and you can earn credits as well.