Exactly that’s the reason - you don’t have a device/app assigned. 

Devices/apps use up RingEX licenses. 
Without a RingEX license you are basically a free user who has access to message/video and can be set to be an administrator, but very limited access to telephony. 

The phone number you got is an additional local number (ALN) and uses up ALN licenses, which are cheaper than a RingEX license. 

Sounds like you need to have your ALN removed, and a RingEX license added instead, or you can keep the ALN alongside the RingEX license,
(RingEX license being synonymous with RingCentral App you mentioned.)

You may also check the ‘role’ of the user you have assigned to that number - depending on the security settings, the ability to call out may not have a checked box.

That’s how we’ve restricted some extensions so that the phone can be in the reception area with only a few preprogrammed extensions reachable. (ala limited +)

Thank you @Joe Cache & @Problem Solver 

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