All numbers should follow the international format; for that, you need the country prefix. It's really important for all numbers to follow this format, without it, you might end up sending to the wrong country!
Add the country prefix, and take off the + and the first 0 from the number:
- Dutch number, country prefix(+31): 0612345678, should be entered as 31612345678
- UK number, country prefix (+44) s: 0712345678, should be entered as 44712345678
Make sure the mobile phone numbers you enter are correct and valid so you don't waste balance on messages sent to non-existent phone numbers.
If you want to add the country codes to all the numbers in your excel file in one go, there is some advice on how to do it here.