SIP accounts and buttons have no correlation between each. You are seeing two line buttons but there is only one SIP account.
https://www.3cx.com/sip-phones/grandstream-gxp21-series/
Read the known limitations at the bottom.
Before version 15 it was only one account showing after ver 16 released and after the upgrade it now showing two accounts .The screenshot info is correct, for the reason posted by cobaltit.
3cx requires two line keys with the same ext number to work correctly.
Thank you for the supportCorrect. Per Grandstream's recommendation on how to work best with 3CX it requires 2 line keys. This was changed in the template so when you upgraded it changed the phones to comply with the latest template. Not a bug but working as intended.
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