Someone

new phone;
new penter;

new PickedUpPhone[MAX_PLAYERS];
new PickedUpEnterToPD[MAX_PLAYERS];

public OnPlayerPickUpPickup(playerid, pickupid)
{
    if(pickupid == phone)
    {
        PickedUpPhone[playerid] = 1;
        // Messages & others
    }
    if(pickupid == penter)
    {
        PickedUpEnterToPD[playerid] = 1;
        TogglePlayerControllable(playerid, false);
    }
}

public OnPlayerCommandText(playerid, cmdtext[])
{
    new cmd[256];
    if(strcmp(cmd, "/enter", true) == 0)
    {
        if(PickedUpEnterToPD[playerid] == 1)
        {
            SetPlayerInterior...
            SetPlayerPos...
            // Basically now You put the player in PD
        }
        else
        {
            SendClientMessage(playerid, COLOR_RED, "You must be in a checkpoint OMG!");
        }
        return 1;
     }
     return 0;
}

// And when You'll make more entrances...

new phone;
new penter;
new mallenter;

new PickedUpPhone[MAX_PLAYERS];
new PickedUpEnterToPD[MAX_PLAYERS];
new PickedUpEnterToMall[MAX_PLAYERS];

public OnPlayerPickUpPickup(playerid, pickupid)
{
    if(pickupid == phone)
    {
        PickedUpPhone[playerid] = 1;
        // Messages & others
    }
    if(pickupid == pdenter)
    {
        PickedUpEnterToPD[playerid] = 1;
        TogglePlayerControllable(playerid, false);
    }
    if(pickupid == mallenter)
    {
        PickedUpEnterToMall[playerid] = 1;
        TogglePlayerControllable(playerid, false);
    }
}


public OnPlayerCommandText(playerid, cmdtext[])
{
    new cmd[256];
    if(strcmp(cmd, "/enter", true) == 0)
    {
        if(PickedUpEnterToPD[playerid] == 1)
        {
            SetPlayerInterior...
            SetPlayerPos...
            // Basically now You put the player in PD
        }
        else if(PickedUpEnterToMall[playerid] == 1)
        {
            // Basically now You put the player in mall
        }
        else
        {
            SendClientMessage(playerid, COLOR_RED, "You must be in a checkpoint OMG!");
        }
        return 1;
     }
     return 0;
}