[Admin Tool Feature]: Phone value as string instead of number

[ssandino]

Is there an existing request for this feature?

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Is your feature request related to a problem? Please describe.

Proposition by @mkue in a merged PR #706 :

I think we should switch to using a string to store a phone number using the + prefix. The validity of the phone number could then easily be checked with a regex.

@MDikkii @novas1r1, can you assess the impact of such a change on the mobile app? Are there any best practices in mobile development that should be considered?

Describe the solution/feature

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[mkue]

Use libphonenumber for serialization and de-serialization.

Communicate with mobile app team before deploying the changes.