Born in Salford, near Manchester in northwest England, Phillips -- Welsh-qualified through a mother from Swansea -- was capped by Wales at Under-16 level in 2021 before going on to make over 30 appearances for England youth teams.
Phillips was approached by Wales to rejoin their national set-up, but his name was missing from manager Craig Bellamy's 26-strong squad announced Tuesday for the play-off semi-final against Bosnia and Herzegovina in Cardiff on March 26.
The 20-year-old is on loan at Stoke and has made over 70 appearances across two spells for the English second-tier club.
The winners of the Wales-Bosnia tie will face either Italy or Northern Ireland at home on March 31 for a place at a World Cup in Canada, Mexico and the United States that runs from June 11 to July 19.
Injuries to senior players have disrupted Wales' play-off plans.
Wales captain Ben Davies, also with Tottenham, broke his ankle in January, and the defender, who won his 100th cap in October, could miss the World Cup should the Dragons qualify.
Centre-back Chris Mepham and striker Kieffer Moore are both missing out on the Bosnia tie because of hamstring problems.
Wrexham's Danny Ward, one of four goalkeepers in the squad, has been recalled after dislocating his elbow and spending four months on the sidelines.
Full-back Rhys Norrington-Davies and winger Rabbi Matondo, who has not started a game at Scottish giants Rangers this season, also return to the squad.