[biker122]

#EDIT:
Believing that the phone number is a numeric value, and is NON-ZERO - this code should work.

pawn Code:

if (strcmp(cmd, "/phnumber", true) == 0)
{
    new string[40];
    new phnumber[20];
    if(PlayerInfo[targetid][pPnumber] == 0) format(phnumber, 20, None);
    else if(PlayerInfo[targetid][pPnumber] != 0) format(phnumber, "%i", PlayerInfo[targetid][pPnumber]);    
    format(string, sizeof(string), "Your phone number is %s",phnumber);
    SendClientMessage(playerid, COLOR_GREY,string);
    return 1;
}

The code is untested, yet it should work fine.
I have changed '%d' to '%s' while formatting the 'string' because, phnumber is a string and not a integer/numeric value.