Rework phone number format

[vzahradnik]

While I worked on GPRS support, I noticed that all examples and libraries use phone number with international extension, including the + sign. Adeon doesn't use this sign, which makes it incompatible with the rest.

A phone number starting with a country extension without a plus sign, e.g., 420xxxxxxxxx, is invalid. A valid format is either "+420xxxxxxxxx" or "00420xxxxxxxxx".