7723 is apparently the number that the Google/Android dialer app sends an SMS to to check Visual Voicemail status. Maybe it's a leftover setting if you previously used the phone on a different carrier? Maybe you can find and disable any references to Visual Voicemail in the phone app settings. Also try a network settings reset. Worst case, a full phone reset.
Lastly, quoting @srlawren from a previous post "Revoking the SMS permission on the Phone app seems to prevent it from doing so. As a PM customer, there is no downside to revoking this permission, since VVM is not a service that's offered here
