problem calling between echo devices

[fivade]

Hi everyone. A few people in my family have amazon echos and we sometimes call between them. Normally, I just say 'alexa, call <name>' and it calls the other persons echo. all good.

however, I'm noticing that more frequently now when i do that, alexa will say 'I can't call a mobile'.
the contact is in my contacts list with a mobile and an email address - i'm not sure which alexa uses to know if the other person has an echo.
sometimes, if i repeatedly ask alexa to make the call, it will eventually do it, calling their echo.

Any ideas why it will sometimes say it can't call a mobile ? or is there a specific phrase I should use to call another echo to tell alexa that I don't want it to call mobiles. i have tried several variations, including 'call <person's> echo device'
but it will still say ' i can't call a mobile'.

(note that I'm based in the UK, and so is the person I'm attempting to call )

thanks for any suggestions as it's getting quite frustrating!