Any number of things that could be going on here. Most likely it is a software setting that may have changed with out your noticing or even knowing it exists. Software interfaces can be unnecessarily complex and cryptic. If you are using an email reader on your tablet, that app may have updated itself, which possibly changed a setting. So much fun to be had with technology!
You are correct in that the CapTel phone should not
have any impact on your email.
The Sonic team are right to urge IMAP email configuration when using more than one device to view email. It is best to leave email on the server (ie. sonic.net) and have your Mail apps retrieve msgs on-demand. I usually have people set their software to "leave on server" unless deleted, and even then you can often retain deleted messages for a set period of time, allowing you the chance to get it back later if accidentally deleted.
- The phone Mail app may be automatically moving msgs. after they are viewed (possibly to a hidden folder).
- The desktop Mail app may be ignoring (not showing) msgs. marked as viewed.
Regarding your Windows XP system. I am sure you have heard that Microsoft discontinued support for XP back in April 2014. So it no longer gets security updates which can be a real problem as time goes on. If you must use it for checking email, I strongly urge you to ONLY use it to connect to Sonic.com email, and do not view or download any photos/attachments unless your life depends on it. Sorry to turn the screws of fear, but older systems that are off support can be a gold mine for bad people to take advantage of. Sonic webmail has a setting to NOT show images by default and you might want to have one of their team help you check that. There is a button you can click to show images in any email you receive, so it just gives you a chance to decide.