The phone number is incorrect because you're not using "+" symbol or two zeroes (00) in the beginning. If you're sure that your phone number does not require a prefix next to it, then you can edit this in the config avoiding the validation.
You will have to edit the file located in /etc/config and called sms_utils and edit it with this part in it:
use the phone number you need there and save the file. Once you've done editing execute /etc/init.d/sms-utils restart command and everything should reload with the changes you've made.
To edit the file use vi command: https://www.cs.colostate.edu/helpdocs/vi.html
Or any SCP capable file explorer like WinSCP.