Alexa, Learn my voice not working on some devices

[Hex]

I have 4 mobile phones, all connected to the same Amazon Alexa account. Each of these can see the same devices and everything works fine except for the voice profiles.

On 2 of the mobiles, if i say to alexa (via the mobile or device) "Alexa, learn my voice", it works and starts the process. As soon as I try this on 2 of the other devices (or mobiles), I get the typical error of "At this time, i'm having trouble learning your voice".

I've checked these mobiles, and they all look to be identical. I assume it must be a location issue but can't figure out where to find this as I've compared phones and settings within the Amazon Alexa app.

Could someone PLEASE help, i'm going crazy here!

Thanks