Forms :: Can't Add Data To Subform

I had a problem earlier with a subform (posted May 30/2013, "Subform is blank") whose database was fairly complex (for me, at least), so problems I had with the subform may have had many causes. So I decided to build a very simple test database for employees and their phone numbers. There are three tables: AllContacts, for the names, PhoneNumbers for the phone numbers, and PhoneNumberType for the type of number (office, mobile, home, etc.).

Each employee may have several phone numbers. A main form is for entering firstname and lastname, and the subform is for entering the phone number(s), and the type of number (office, mobile, home, etc.) for each one.

The problem I'm having is that the subform will not accept data in its Number Type field. The error is "The field cannot be updated because another user or process has locked the corresponding record or table. (Error 3164)." I'm the only user, and the control has no locked fields. The Phone Number field will also not accept data: "Cannot add record(s); join key of table 'PhoneNumbers' not in recordset."

I suspect my table relationships may be incorrect, but I don't see where.