I'll comment on a few of your concerns. You didn't mention which phone you have, but I'll assume it's not an iPhone.
1. That's not an Xfinity Mobile problem. If I'm not mistaken, any mobile phone needs to be connected to one of those three signals to receive voicemail. Were you previously able to receive voicemail from another provider without connecting to some sort of signal?
2. Some apps use data in the background even if you're not actively using them. Turn on your data saver setting. See #3 at https://www.xfinity.com/hub/mobile/4-cell-phone-settings-you-should-be-using?pc=1
4. You should keep tabs on your data usage yourself through your Xfinity Mobile account or the Xfinity Mobile app. You're only notified for every 5 GB of shared data used. See https://www.xfinity.com/mobile/support/article/221763947/how-do-i-track-data-usage-for-by-the-gig-li...
Mobile-Informed, rightfooted made some good points.
As far as your signal reception, are you using the pocket geek app? If you are, I can check out your signal concerns through that app. Is your area within our coverage zone?
Did you have to buy a phone from us because your BYOD device was not compatible? What tech problems are you experiencing?