SMS
We are interested in adding the SMS add on but before you fix the phone number issue it will not be worth it.
In our previous system they separated the country code from the nr. just do it like that. So we choose the country code from the drop down menu and then write the number after. It should be easy even to integrate with different channels - I know it is possible with booking.com and Expedia.
I think when making a reservation there are some things that are basic: name, telephonenumer and email adress. That there is an issue with the phone number seems unneccessary and critical to fix.

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Tea Westelius commented
I think its time to fix the phone number issue that seems to have been ongoing since the start of Mews. I get that it is not a critical "problem" but it is annoying and IS making us choose not to add on the SMS integration to our property.
If you have required format for the phone number for your own SMS service why not help the properties get the numbers right instead of forcing us to make the changes manually - seems against your efficiency and automation vision.
Just make a drop down menu before the number for the country code and then the number will be written after.
Standard way of handling it and I know it can be integrated with for example booking.com as our previous system handles it like that. Additionally that is how booking.com handles it originally.
Even if its on the "Not important" - "Not urgent" part of the Eisenhower matrix it needs to be fixed.
First impression after my onboarding was that it was a bad sign that you "couldn't even get a basic feature right" when I saw how the phone numbers were handled. Which is pretty unneccessary thought for a new customer to have.