Text Messages Using gmail Contacts

I use gmail exclusively, and I have a "group" made up of some of my gmail contacts. Some have an email address and a cellphone number, and others have only a cellphone number.

I wish to send a group text to everyone for receipt on their cellphones. The contacts with a cellphone number receive the message on their cellphone as desired. However, the contacts with  both an email address and a cellphone number receive the message as an email, and not on their cellphone.

Any suggestions on this?

Thanks in advance!