Yeah for sure.
Roaming is when you use another operator's core network. You'll need to enable roaming on your phone in order to get service.
In a RAN sharing scenario, you will always use your own operator's core network. You can disable roaming on your phone and all your services will work.
Let's look at an example.
A Telus (Public Mobile) customer in Toronto will connect to Bell's tower (eNB), and that eNB is connected to Bell's core and Telus's core. During the attach procedure, the phone will request to the eNB that it wants to connect to Telus. Therefore, Bell eNB will ask Telus core (MME) to authenticate/register this user. If Telus core (HSS) accepts this user, it will be allowed to make voice calls and use the internet as allowed on the user's account. The user plane connection will go from Bell eNB to Telus SGW to Telus PGW to internet.
If a Telus user goes to US and tries to connect to an AT&T eNB, the eNB will ask the AT&T core (MME) to find this user's roaming capability. AT&T MME will ask Telus HSS if this user is allowed to roam, and if so, it will authenticate this user and use AT&T eNB, AT&T SGW and Telus PGW to internet.
Obviously in order for any roaming to work, the 2 operators need to have agreements in place for roaming fees, and connect their cores to each other.
If a Public Mobile customer goes to Mexico and tries to roam on Telcel, the Telcel eNB will ask Telcel MME to ask Telus HSS if roaming is allowed. Telus HSS will reply that roaming is not allowed, and the attach procedure will be rejected. The user will only be allowed to make emergency calls, and all other services will be denied. The end user will not know if Telcel is lacking roaming agreements with Telus, or if Telus simply isn't allowing this particular plan to roam in Mexico. In reality, since Telus branded customers can roam in Mexico, we know the agreements exist, but Telus has made a business decision to reject roaming outside of US.
