Check that telephone numbers that are not visible are registered with a prefix (+352, +33 or +32)*, and that there is only one number per contact. Your contact must also have finalised registration of their telephone number with one of their accounts to be visible. If no contacts appear in the list, the app may not be authorised to access your contact list. See next question to check this.
*Payconiq allows you to send payments to BeNeLux, so you need to register your contacts with international prefixes (+352, +33 or +32, etc.) for them to be visible.