Before you get too far down the road, check to make sure you have SIP trunk licenses in system-parameters customer-options. With that said, you have received some wise counsel to not expose your switch to your provider's SBC. You really should have System and Session managers and your own SBC. Without them you will be limited on what you can manipulate in your SIP traffic and will be trusting your provider to protect you. You might also struggle during initial setup and future troubleshooting because of CM's list trace feature not being as good as ASM's traceSM/traceSBC. If all of the above aligns for you, there's no other technical reason it won't work.
We're all ignorant, just in different subjects.