I think it come from myself, but thank you to small function usage here: my first reading of this line was cause of reflexion to understand logic and thinking there was a issue:

• testing against an empty value to use the testing value (the empty one)
• if not empty: use it too....

🤔
Then I realized the final dot for second case; then use to format a string
Just a personal thought I think I would have written logic as
const formattedRegion = region !== '' ? `${region}.` '';
Which could highlight in a better way the logic and use case.
Again just a personal thought and not a blocker

You are right, it would be more readable.

Fixed for all 4 APIs that use regions

By inverting the point of view; could the type being more user friendly ?
I mean:

export enum SupportedSmsRegion {
  UNITED_STATES = 'us',
  EUROPE = 'eu',
  BRAZIL = 'br',
  CANADA = 'ca',
  AUSTRALIA = 'au'
}
export type SmsRegion = SupportedSmsRegion | string;

So when function prototype or variable usage will reference the type, it will be named SmsRegion.
Seems to be more "user friendly" than a SmsRegionFlexible reference and usage.
What do you think?

This would be more understandable indeed. The issue being that with this implementation, I can't do SmsRegion.EUROPE as SmsRegion refers to a Type, but would be used as a value
A solution would be to trick the user and define on top of the enum a new object SmsRegion such as

export const SmsRegion = {
  ...SupportedSmsRegion,
};

WDYT?

As usual, we should have end user and SDK usage in mind first.
so if a SmsRegion type is identified as the best way to reach this objective, if by implementing it as you described in your comment above: it is very fine
Then, using same pattern for Conversation, if SDK usage enable to store a variable/use a parameter type named ConversationRegion and reference ConversationRegion.UNITED_STATES or use us directly : it should provide a better DX (IMO)

Ok, we can do that. Keep in mind the trick I mentioned above: ConversationRegion.UNITED_STATES will not reference the enum, but an object of which we read the value associated to the key UNITED_STATES 😉

Fixed