Adding additional SIP accounts:
To add additional SIP accounts you need Zoiper5 Pro.
Note: Zoiper5 Free (Community edition) is limited to just one SIP account.
After you have set your primary SIP or IAX2 account, additional accounts can be added by going to Zoiper5 → Settings → Accounts → "Add".
This would open the SIP account setup wizard.
Repeat the steps to add all your accounts.
Zoiper5 Pro does not have any limit on the number of SIP account which can run concurrently.