My iPhone's number won't add itself to iMessage or FaceTime!

My new phone number is greyed out and not available to be selected for iMessage or FaceTime. I've changed my iMessage/FaceTime phone number in the past without any issue, but now this new one won't add! I've tried logging out of ALL devices and logging back in, but it didn't help. No matter what I do, I can't select my telephone number to be active for iMessage/FaceTime!

Here's some background:

1. First phone number on an iPhone 3GS with AT&T: Phone number worked fine on iMessage!
2. Took my iPhone 3GS (unlocked) to Straight Talk Wireless (a Tracfone prepaid service branded for Wal-Mart) with a NEW number: Phone number worked fine on iMessage!
3. Upgraded to iPhone 5 (locked to T-Mobile) on Straight Talk Wireless (with T-Mobile-compatible SIM) with the SAME Straight Talk Wireless phone number: Phone number worked fine on iMessage! This newer iPhone also introduced FaceTime and my number worked fine on that too!
4. Took the same iPhone 5 to Brightspot Wireless (a T-Mobile prepaid service branded for Target Stores) and got a NEW phone number: This new number will NOT add to iMessage or FaceTime no matter what I do! It stays grayed out and cannot be selected!

I have other Apple devices that also use iMessage and/or FaceTime:

• iMac on OS X Yosemite Beta with both iMessage & FaceTime
• iPhone 4S on iOS 8.0.2 used only as a glorified iPod Touch (kept on Airplane Mode with WiFi only -- it has an inactive dummy SIM so it would activate in iTunes and operate normally) with both iMessage & FaceTime
• iPhone 3GS on iOS 6.1.3 (the same as used before with Straight Talk Wireless) now only used as a glorified iPod Touch (kept on Airplane Mode with WiFi only -- this one still has its most recent Straight Talk Wireless SIM in it so it would activate after its restore in iTunes and operate normally) with iMessage only (no FaceTime)

In case it matters, the iPhone 5 in question (with the phone number that won't add) is on iOS 7.1.2.

People have my new number but it's not going to all my devices when another iDevice sends me an iMessage and it's really frustrating for this to not work properly! My only thought is that my iPhone 5's custom APN settings keep erasing themselves after a restart or a loss in signal, causing me to have no MMS or data (except WiFi) until I type the settings back in. I can see why this might make me temporarily disconnected from Apple's servers, but I don't see why it won't add the phone number in the first place! Please help!