Checking numbers is cheaper.
Each check (or recheck) only costs 0.5 credits. Even more of a reason to use this service, so you don’t waste credits sending messages to number which cannot receive them.
Here are the relevant Help Desk articles with more details:
Auto-Check Undelivered Messages
How to check your contact’s phone number information / classification (HUB)
How to check your contact’s phone number information / classification (Legacy)