Koodo's main roaming partners in the US are T-Mobile, which still supports some older 3G connections and AT&T, which requires VoLTE (voice over 4G, often marketed as high definition calling).
Koodo supports VoLTE but only for certain phones and NOT AT ALL for pre-paid customers. The phone models supported by Telus and the US carriers don't overlap 100% either. I headed south with an older LG V20 and a more modern LG Velvet which BOTH AT&T and Telus support.
In an area where T-Mobile has good service (Philadelphia airport), the older Koodo phone performed OK, but that only built some false confidence and expectations.
Landed further south, where AT&T is more common, both phones would kick in, then post a notification that "services are temporarily not available" every 10 seconds or so. This bouncing back and forth may explain why some users are complaining of phantom roaming charges near the border. The only way to get any peace was to shift to airplane mode, which defeats the whole purpose of roaming.
Koodo should consider blocking AT&T roaming on prepaid entirely, since even if you have a 100% compatible phone, AT&T roaming will not work. Customers should also be aware that roaming signals may be de-prioritised by US carriers. At first it was a struggle to get a usable signal except in dense urban settings. That problem magically disappeared when I bought a prepaid SIM and plan from AT&T.
Needless to say, the US plan is full-featured and less expensive than the equivalent Canadian one.