Roberts, 22, was visiting friends at West Rugby Club in Canberra, where he played last summer on release from the Blues, when Phillips tracked him down and he joined up with the squad today.
Horsman will be assessed once he arrives in Australia and, although he is fully fit and there should be no problem with him answering his country's call on Saturday, there is still an
outside chance Roberts could be involved.
"I'm just extremely happy to be here," said Roberts, who has played for both the Cardiff Blues region and Cardiff Rugby Club this season and represented Wales in the U21's World Cup in Argentina in 2005.
"I'm getting stuck into training and just trying to enjoy myself. It's difficult to believe I'm here really.
"Last week I was in Canberra watching the game on TV in West Rugby Club, with my friends joking about me being called up and the next thing I know the management are handing out kit and giving me a squad number.
"It's pretty unreal, but I just want to do everything I can to help out. I'm not thinking about the outside chance of me being involved on matchday, its just enough for me to be in this environment and to be able to do something towards the national cause."
Jones pulled up in the second training session of the day yesterday (Wednesday) and an ultrasound scan revealed the extent of the injury, forcing him to withdraw this morning.
Fellow tight-head prop Jamie Corsi has also been ruled out, after a back injury sustained in the gymn last week failed to clear up in time, leading Wales to call for Horsman to fly down under.
Ceri Jones, who can play on both sides of the scrum, takes his namesake's place in the front row and Horsman will be asked to provide cover from the bench when he arrives in Brisbane at 10.30am tomorrow (Friday) morning.
"Adam has a small tear to his right calf muscle which will prevent him from training now for three to four weeks," said Wales team doctor John Williams.
"Unfortunately, Jamie sustained a lower back injury in the gymn last week which has not cleared up in time to make him available for selection and so we have arranged for Chris to fly out.
"The protocol for him playing rugby after a long haul flight like that will need to be managed and we will be giving him all the correct advice when he arrives.
"Jet lag shouldn't be a major issue, but we will try to get him to sleep in as much as possible on matchday as tiredness will probably be his main problem if he has one.
"The actual effects of the flight on his body clock probably won't have had time to affect him because the game comes so soon after he lands and takes place at 11am UK-time."
Adam Jones has backed his former understudy to rise to the challenge presented by Australia on Saturday night.
"Ceri will go alright, there's no worries about that," he said.
"He went well when he came on last week and he's got what it takes to handle the challenge on Saturday.
"I'm obviously gutted to be missing out, but I'll be wishing Ceri all the best before the game and I'll be right there for him if he needs anything from me."
Chris HORSMAN (WORCESTER WARRIORS)
DoB: 02.02.78. Ht: 6'3"/1.91m. Wt: 17st 6lbs/111kg
International record: 9 caps
Scott ROBERTS (CARDIFF BLUES)
DoB: 05.09.84. Ht: 5'10"/1.81m. Wt: 17st 0lbs/108kg
International record: Uncapped