If that's the number in the user guide, and it does not work, then try to contact UM's customer service.
Quote:
Originally Posted by teothx
It doesn't work. The phone says "number invalid". 
But if I call the 077 it says some options but not to switch off the voice mail...  |
Phones: Xiaomi Mi Mix 2, Samsung Galaxy A50, ASUS zenfone 3,
Sim cards: AT&T (Contract), 3 UK, Piranha Mobile