phone in a textbox

[arale]

ok i have a textbox that the user is filling a phone number in it .
i want to block writing letters and if im writing :

if isnumeric(txtphone)= false then
msgbox "incorrect number"

else
endif

but i cant write ( - ) to separate the areacode from the number :
954 - 65823154 what can i do without putting an extra textbox for the areacode?

thanks