Combine Columns From Two SELECT Statements

I have a database that tracks billing and payment history records against a "relationship" record (the "relationship" maps a many-to-many relationship between employees and cell phone numbers).

I have two statements that look like this:

SELECT CellPhone.PhoneNumber, SUM(BillingHistory.AmountOwed) AS TotalOwed
FROM Relationship
INNER JOIN CellPhone ON CellPhone.PKCellPhone = Relationship.FKCellPhone
INNER JOIN BillingHistory ON Relationship.PKRelationship = BillingHistory.FKRelationship
GROUP BY Relationship.PKRelationship, CellPhone.PhoneNumber

SELECT CellPhone.PhoneNumber, SUM(PaymentHistory.AmountPaid) AS TotalPaid
FROM Relationship
INNER JOIN CellPhone ON CellPhone.PKCellPhone = Relationship.FKCellPhone
INNER JOIN PaymentHistoryON Relationship.PKRelationship = PaymentHistory.FKRelationship
GROUP BY Relationship.PKRelationship, CellPhone.PhoneNumber

Each statement correctly aggregates the sums, but I need a record that shows me:

CellPhone.PhoneNumber, SUM(BillingHistory.AmountOwed) AS TotalOwed, SUM(PaymentHistory.AmountPaid) AS TotalPaid

I can't figure out how to join or merge the statements together to get all of this information into one record without ruining the sums (I can't seem to correctly join the PaymentHistory table to the BillingHistory table without the sums going haywire).

Any help is appreciated.