Bruce,
Why do you want to do this? For one thing, this is a derived value, and as such it should not be stored, and instead be calculated on demand. For another, now whenever the acct number or phone number changes you must also update that combined column.
If the column will not be unique, then it cannot be a primary key; if all you want is an index, you can create an index, unique or otherwise, that comprises the two columns:
http://en.allexperts.com/q/Using-MS-Access-1440/Creating-Unique-Value-check.htm
(That covers a unique index, but you use similar work steps to create a non-unique index.)
Patrick