How to get a popup with the Callee information from SuiteCRM, when someone calls the agent

I have no experience with SuiteCRM and there is no official integration for SuiteCRM from RingCentral.

If you want to do it yourself, I think it is possible, provided you have good knowledge of SuiteCRM. Whenever there is an incoming call, RingCentral API can send you a notification then you can look up the caller information in SuiteCRM.

SuiteCRM is a fork of SugarCRM and there is an out-of-the-box integration with SugarCRM and RingCentral by Tenfold:

https://apps1.ringcentral.com/app/312709020-481791020-7gufiGT3T3CCuCP37hMDaQ~fTR6MCGsTuKxZ8tyhAFAcA-...

I'm not familiar with how this integration is done but perhaps you can either get some info via that integration or ask them to support SuiteCRM.