Hi, Its possible to terminate SIP Trunk directly to CUCM. You do not need any special router between.
But I think you would like to consider the security side and not expose the CUCM to exploitation.
I would definitely deploy some kind of Session border element between CUCM and ISP which I could monitor and configure to avoid any misuse of the SIP Trunk.
ISP will be giving you username passwords for the SIP trunk for its authentication, which i don't think you can configure in CUCM. You have to use Cisco router with CUBE session licenses along with UC package. Or any SBC for this purpose.
If you are comfortable with Cisco you should use Cube.
I have found sometimes convinient for me Audiocodes Mediant and those Opensource SBC’s like on my first comment.
All of those would work, and would have all normal features one would require for basic SIP trunking use.
I recon as you are searching help here maybe would be good to contact some vendor or such to make the deployment for you? I guess its quite a steep learning curve to get that kind of job done from zero.
Just to add to the other members' replies, if you are going to use Cisco CUBE, the routers that it will run on would be an ISR2900 (2911, 2921, etc.), ISR3900, ISR4000 (4300, 4400, 4321, 4331, 4451, etc.), or even a CSR1000v, I personally do not recommend the CSR1000v because it is not currently capable of transcoding.
If you are doing any transcoding, you will need PVDM modules aka DSP modules, but if you are simply terminating a boundary, then DSPs may not be needed. The biggest reason for the need of transcoding is codec mismatches, say for example your service provider only supports G711, but you have phones running G729, then you would need transcoding. There are other reasons for the need of DSPs/PVDMs, but it doesn't sound like you need that. And as stated, you will need CUBE session and UC licenses. If you have any experience with Call Manager Express (call manager on a router), then you have at-least a beginners idea for configuring CUBE.
If you are connecting to your SIP provider over the public Internet, be very careful and be sure to include security configurations such as access lists, etc. to prevent unwanted security risks.
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