Phillips, 33, who spent two seasons at Southampton between 2003 and 2005, rescued a point for Albion as his 13th goal of the season in first-half stoppage time cancelled out Kenwyne Jones' 18th minute opener.
Albion manager Tony Mowbray had been desperate to get his hands on the three points - and the bragging rights - so he could put one over his Southampton counterpart and mentor, George Burley.
It was Burley who launched Mowbray's coaching career when they were both at Ipswich Town.
Albion certainly had enough chances at the start of the second half to have secured a seventh successive win as Zoltan Gera, Jason Koumas, whose shot was cleared off the line by Gareth Bale, and Paul McShane were all frustrated in front of goal.
Having weathered that storm, Southampton will also consider themselves unfortunate not to have taken all three points from the game as they stretched their good run to just one defeat in 10 games.
Albion goalkeeper Dean Kiely, who came under fire from the Southampton fans following his spell with their arch-rivals Portsmouth, pulled off two impressive saves in quick succession to deny Rudi Skacel and then Andrew Surman in the 72nd minute.
Just 60 seconds later, the Saints bench was calling for his dismissal after claiming he handled the ball outside the area to deny Jones only for their appeals to be waved away by referee Mike Pike.
Southampton were well worth their lead after stifling Albion's normally free-flowing play in the opening stages.
The Baggies failed to heed a warning two minutes before Jones struck when Skacel shot just wide.
They didn't escape a second time as the vision of John Viafara cut the Albion defence apart. His raking throughball was collected by Grzegorz Rasiak whose neat cross was rifled past Kiely from six yards by the unmarked Jones to take his tally for the season to 12.
Albion, who had been chasing a ninth successive home win, struggled to come to terms with the shock of falling behind and it was only in the final five minutes of the first half that they came to life.
Gera fired a snap-shot wide before Phillips showed his predatory instincts to equalise. The former England marksman was the fastest to react from close range when Koumas' misplaced shot found its way into his path after McShane's cross had caused panic in the penalty area.