Are you using CUBE to talk between CUCM and your service provider? If so, you need to purchase a CUBE license. FYI, if your provider wants early offer for outbound calls (they typically do) then you need a SIP-SIP CUBE to achieve that. Hence, the majority of SIP ITSP trunks to CM require a CUBE for this reason.
If the SIP trunk is CUCM directly to the ITSP, no licenses necessary, since it is just going to look like any other SIP trunk to a device off of CM.