@DeborahHenry Your carrier has to support it first of all, sounds like that did previously.  Also what version of iPhones?  Do yo have any other H/W, S/W or VPNs in place that might be interfering with it?  Also try restarting the iPhones  Mine says "T-Mobile WIfi" all the time and I have 3xXE5300 Deco Units. 

  @DeborahHenry the latest firmware version for this router is actually V2_1.2.9 Build 20240724.  You can download it from the product page. Having the wrong firmware version can cause the issue you're seeing. 
 

Also, makes sure your iPhones have the latest version of iOS, and reset the network settings on the iPhones - Settings>General>Transfer or Reset iPhone>Reset>Reset Network Settings; if that doesn't work, you need to contact your carrier to see why they disabled wireless calling on your account.