I have some rows in data where agentid and customer ids are being repeated. like so

I want to assign a row number to each of the repeated value so that the result looks like this table below: Table 2:
| agentid | customerid | row_number |
|---|---|---|
| 26 | 1234 | 1 |
| 26 | 1234 | 2 |
| 26 | 1234 | 3 |
| 26 | 1234 | 4 |
| 26 | 1454 | 1 |
| 26 | 1256 | 1 |
| 26 | 1256 | 2 |
| 30 | 1256 | 1 |
I am new to PostgreSQL, so I tried this:
select agentid, customerid, row_number() OVER () from aa_dev.calls group by agentid, customerid having count(customerid) > 1;
This query above selected all the repeated pairs and assigned rows to each of them

but I want to create something that I mentioned above in table 2. Please guide me about this.