Introduce number validation in SDK

[atsakiridis]

Consider using this handy lib: https://github.com/googlei18n/libphonenumber

[atsakiridis]

This is becoming more important as demonstrated by importing contacts from the device

[atsakiridis]

Also keep an eye for @deruelle's comment:

also we need to be careful as we can call SIP numbers that are a bit different than PSTN numbers
so if the validation is too strong
we won't be able to input SIP Numbers that are different from PSTN numbers