The reason why you are not able to receive texts from some but can receive from others is likely because the texts are not sent as SMS messages. Text messages can be sent in different forms. The most common are SMS, MMS, RCS, and iMessage. Tobii Dynavox currently only accepts SMS (with the exception of Beam which also accepts MMS).
You can ask the sender to send the message as SMS by asking them to change their settings. The instructions below may help the sender make the change however iPhone and Android screens do tend to change over time so for the most accurate instructions you may need to check their websites.
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