Standard Sport understands that while a deal is not easy to do, Middlesbrough would prefer a permanent move, but are also open to a loan for the 21-year-old.
Middlesbrough have already completed the signing of Spurs striker Will Lankshear for a fee that could rise to £20m if add-ons are met, and they are keen to raid the north London club again to sign Phillips.
Phillips has established himself as one of the brightest young centre-backs in the Football League, having made more than 100 Championship appearances across loan spells with Stoke City, Plymouth Argyle and Blackburn Rovers.
The England youth international made 40 appearances in all competitions for Stoke last season and captained the side in their 3-3 draw with Ipswich Town in March.
Standard Sport understands that Spurs want to drive greater productivity from the club’s academy, whether through first team progression or player sales.
They have already raised up to £34m from the permanent departures of Lankshear, Alfie Devine and Tynan Thompson, while teenage defender Luka Vuskovic joined Brighton for £50m despite not making a single competitive appearance for Spurs.
There is also an acceptance that overall Spurs need to improve at maximising player sales. After an aggressive summer of spending, in which they have paid in excess of £200m for six new players, academy sales may increase as the club looks to generate funds.