If you have multiple channels of the same type connected to your account, the channel that the contact last initiated contact with will be prioritized. Contacts can be locked-in to a relationship with a specific channel (they'll only receive messages from this channel) under three conditions:
You have multiple channels of the same type connected to your account.
The contact's address type priority corresponds with the channel type (e.g. phone number) that possess multiple channels (e.g. multiple phone numbers).
The contact initiates contact through an incoming message.
If the contact has never interacted with a channel linked to your account, we will prioritize the channel whose number has the largest prefix overlap (starts most similarly).
For example, if you have an Android Relayer Channel that is +12505661212 and one that is +2505551212, and you are sending an SMS to or calling +2505661231, we will use the +2505661212 Channel because it overlaps the most.
In most cases, we are able to discern the carrier associated with a contact's phone number and prioritize channels based on carrier, though this depends on carrier behavior.
You can use the Set preferred channel action to either assign or switch a contact's preferred channel without waiting for a response.