In the call which was made public on the internet on Tuesday evening, a male who appears to be the Spaniard tells a man posing as an American-based agent details of the four-year deal he signed with the Ibrox club in the summer, including a breakdown of his salary which increases year-on-year to £10k-per-week in his final season.
The recipient of the call then expresses a desire to leave Ibrox at the end of the season, citing the standard of football in the bottom tier and the weather after being seemingly offered a move to the MLS.
If it is found the call is indeed Sandaza, the former Dundee United and St Johnstone striker could find himself in trouble with the SFA, whose rules state that no player should attempt to negotiate a transfer while under contract.
In a statement Rangers said: "The club is aware of this issue and is investigating the matter.
"Ally McCoist has spoken with the player and Rangers Chief Executive Charles Green is consulting with the club's advisers. The club regards this as an extremely serious matter and will comment further at the appropriate time."
Meanwhile, Middlesbrough winger Andy Halliday has revealed he turned down a move to Ibrox to pen a new long-term deal with the Championship club.
The former Livingston youngster had been linked with a Bosman move to his boyhood heroes at the end of the season but has opted to extend his stay south of the border, signing a contract that will keep him at the Riverside until the summer of 2015.
Halliday said: "I don't hide the fact that I am a massive Rangers fan. My family are all Rangers fans and my first flat was right next to Ibrox.
"There was a bit of interest from Rangers I think, but I am really enjoying myself at Middlesbrough and, what can I say, it is an excellent place to play my football.
Meanwhile, tickets for Rangers' visit to Queen's Park on April 7 are on sale from today, with adults £16, juveniles £1.