if branch phones register with HQ cucm and if this router act as just a gateway for terminating PSTN , you need CUCM license based on the phone model/user and for voice feature to work on Gateway you need UC license on router.
If you need to configure SRST feature on the same router , your need SRST license too. With Flex Enhanced license SRST is free.
Cube license will be required if you need Cube feature when subscribed for SIP trunk from ISP. When using SIP trunk, Cube is highly recommended.
I would suggest to work with a partner who is specialized in voice.