Q: iMessage issues - Cannot iMessage to phone numbers

I can send iMessages to the *phone numbers* of all my iPhone user contacts with the exception of two. Both are marked as iPhone next to their phone number in my contacts, their e-mail address that is registered to their iMessage account is also under the same contact info.

I have checked the settings of my iPhone and theirs. Under addresses they can be reached at both have their email address and cell phone numbers checked off. Under "start my conversations from" their phone number is selected. The same is the case for my own iPhone in both instances.

I can send iMessages to their email addresses. When they message me, they come through from their email addresses. When I send to them as text, they get it as an iMessage.

Still, I cannot send iMessages to their cell phone numbers.

Why?