For our international users wanting to use SMS notifications and to invite family members using mobile numbers, please use E.164 Formatting.
For example, here's a UK-based number in standard local formatting: 020 7183 8750
Here's the same phone number in E.164 formatting: +442071838750
For the number above, you would want to input +442071838750 in order to work with Seesaw properly.
You can find more information about international formatting here!