Please sign out, restart the app, then sign back in to your account and try verifying your mobile number again.
In the mobile verification screen, please select your country/location from the "Country" list, then type your phone number WITHOUT the country code. Note that the country code is auto-filled depending on the country you have selected.
It usually takes a few seconds to receive our verification code, and note that it could come from an unnamed number. The message would say something like:
P-0000 is your verification code for Hello Pal
If it still doesn't work for you, please contact us at firstname.lastname@example.org and reply with your mobile number and country code so we can troubleshoot further.