The singer-and-actress - who filed for divorce from the former Fall Out Boy rocker in February after two years of marriage - is said to have made a late night phone call to Pete to tell him how much she "misses" him.
A source told Us Weekly magazine: "Ashlee was telling Pete how much she misses him and wants him back."
The 'Outta My Head' hitmaker - who has a two-year-old son Bronx Mowgli with Pete - has apparently asked her sister Jessica to help her win him back.
However, Pete's friends have insisted he will "never" get back with Ashlee, who has been romantically linked to 'Boardwalk Empire' star Vincent Piazza.
A friend said: "He will never go back to her. He's in a good place."
In March, Ashlee was spotted holding hands with her estranged husband, leading to speculation the pair had reconciled.
The singer cited "irreconcilable differences" in a filing for divorce from Pete to Los Angeles County Superior Court, but the pair insisted at the time they were going to "remain friends" for the sake of their son.
In a joint statement released in February, they said: "After careful consideration, we have made the difficult decision to file for divorce.
"We remain friends and deeply committed and loving parents to our son Bronx, whose happiness and well-being remains our number one priority. We ask that everyone honour our privacy as we navigate this next phase of our lives."