For people who are new to GTA Online, here's how contact missions work:
* Each mission has a minimum level requirement to be offered the job.
* Regardless of the minimum requirement, if another player starts a mission and invites you to it you are able to join and take part. Minimum requirements have *some* indication as to the difficulty level, in that bad guys will often be more numerous, have better guns, have more body armour etc at higher levels, but it's not always the case.
* Once you reach the required level yourself there is a possibility that the NPC who gives that specific mission will phone or text you with the mission details, and you can accept to host it and invite other players should you wish.
* Missions that you have previously 'completed' (i.e. taken part in to the end rather than dropped out in the middle of, but it doesn't actually matter if you have succeeded or failed) when you were over the minimum level show up under the Pause > Online > Jobs > Play Job > Rockstar Created Jobs > Missions menu, and can be chosen from there to do a specific mission that you want at any time in the future.
* When you host a job, the game defaults to 'Open'. This means that any random player who requests to do a random job of that type (e.g. Deathmatch, Contact Mission, Survival etc) can get dumped into your job. To change the default to 'Closed' (invite only, so you can invite dMwers or other friends without the job filling with randoms) for every game you start, go to Pause > Online > Settings > Matchmaking. You can still then change a specific job to open via the job start menu if you need/want more players.
The problem is that, if you level up too fast and/or level up by doing a lot of missions that higher level friends invite you on, you can miss the initial text/call from an NPC offering you a specific mission. If you go too far, they end up only offering you the higher level missions, no matter how much you call them up and request a job. As such, i'm missing a few of the lower level missions from my list, and i'm looking to get them added. I got a ton added the other night when I came across a random level 300-odd player who had every mission in the game, but he got bored and logged off before I had a chance to get them all.
The full list of all missions can be found here: http://gta.wikia.com/Missions_in_GTA_Online
If anyone has the following in their missions list, I would really appreciate meeting up online sometime and you hosting the mission so I can get them added to my list. I'm happy to go with people on any specific missions they want as well. I need (alphabetically, as that's how they appear in the Jobs menu):
A Boat in the Bay (Ron), level 40
Artificial Scarcity (Martin), level 19
ATV Steal (Simeon), level 25
Caught Napping (Lamar), level 20
Cops Capacity (Lester), level 19
Pickup Sticks (Martin), level 21