Howard Forums member posted that they had received a T-Mobile based SIM from Consumer Cellular. I was skeptical, as Consumer Cellular has always been an AT&T MVNO that can roam on T-Mobile, but has never offered T-Mobile SIMs. I asked Consumer Cellular if they were now shipping T-Mobile SIMs and got this official reply:
"Our service utilizes two of the largest networks in the U.S., T-Mobile and AT&T Wireless. The majority of our customers are utilizing the AT&T network, however we do have some customers who have been set up on the T-Mobile network, if the coverage in their local area is stronger with T-Mobile.
Having multiple carriers to choose from allows us to provide the best coverage possible based on a customer’s needs. In fact, if a customer wants to be a on a specific network we are more than happy to fulfil their request."
The difference between roaming on T-Mobile with an AT&T SIM and having a T-Mobile SIM is that the T-Mobile SIM will favor T-Mobile's network and use it if it's available and will only roam on other networks if there's no T-Mobile signal and T-Mobile has a roaming agreement with the other network for that particular area.
Looking at T-Mobile and AT&T's native coverage maps (click the maps above to view a larger version) you can see that although AT&T covers a much larger area, there are a few places where T-Mobile has service and AT&T doesn't. The largest area where T-Mobile wins is in north-east Oregon along I-84 between Boise ID and Portland OR. There are smaller pockets where T-Mobile has coverage and AT&T doesn't in central Nevada, along the extreme northern California coast and in Arizona east of Phoenix.
If you're wondering if your Consumer Cellular SIM is AT&T or T-Mobile based, the SIM ID (ICCID) is the surest way to tell. If the SIM ID starts with 890126 it's for T-Mobile, AT&T SIM IDs start with 890141.
To view your SIM ID on iOS go to Settings > General > About and look for ICCID number. Older Android phones displayed the SIM ID in Settings > About Phone > Status but most newer ones don't. You can use an app like SIM Info (free) which displays the SIM ID as "SIM Serial Number". The SIM ID should also be printed on the SIM itself or on the credit card sized piece of plastic the SIM came attached to.
More and more MVNOs seem to be going multi-carrier. Verizon MVNO BYO Wireless recently added T-Mobile to its lineup as did Sprint MVNO Ting. Formerly AT&T only MVNO Red Pocket now offers service on all four national networks. Advantages of nulti-network for MVNOs include a stronger negotiating position with their network partners, being able to server a larger market and to accept more different BYOP devices. For MVNOs like Consumer Cellular that that offer no-extra cost roaming it also means they can reduce roaming costs by providing service on a carrier that the customer is less likely to need roaming with.