hey guys..hope u can help me with this problem...if lets say...i have A's phone number..but i not sure if A got my phone number anot..can i still msg him tru whatsapp? and if A does have my phone number...can i block him from knowing that i have the app?

i re-installed back my whatsapp but for some weird reason..i am not given any notification when i receive a msg..i will only noe when i check it manually..i have the app running in the background..anyone noes why?