Some phones do not get the opt-in request text
It is possible that these phones have blocked text messages from short code(s). In order to opt-In these devices, you can choose the IVR opt-in method. Here is how:
Navigate to the Phone section under the 'Admin menu'
Select the phone(s) (check box)
From the Opt-in list, select 'opt-in via IVR' and hit go (attached screen illustrates the above steps)
This will initiate an IVR call to the selected phone(s). The user needs to answer the call and follow instructions to complete the opt-in process.