mobile number validation

set textbox maxlenght =10
and use
<asp:regularexpressionvalidator id="RegularExpressionValidator1" runat="server"" xmlns:asp="#unknown">
ControlToValidate="TextBox1" ErrorMessage="RegularExpressionValidator"
ValidationExpression="\d+"/asp:RegularExpressionValidator

See here[^].

How many Countrycodes could be entered?

First of all, i would remove all special chars like '-', to get a clean input.
Then i would do a check to the length. If the length is over 10 digits, then its a countrycode included. Then you check again the Regex with Countrycode. And if not, use the second regex.

Pseudocode:

if input.length > 10
  Regex = ^\d{1,2}\d{10}$
else if input.length = 10
  Regex = ^\d{10}$
else
  crap input

See this link

http://www.dotnetfactor.com/Forum/how-to-validate-10-digit-mobile-number-in-Asp-Net-with-regular-expression-control-26.aspx