@IT-ADMIN said:
cuz we dont have a dedicated IT team that can manage this headache of voip, so we decided to delegate this to our ISP, they set up everything for us
That's nothing like having a limitation. You just decided to use the ISP, that's all. VoIP does not take an IT team. There are tons of vendors that will do this for you, either on premises or hosted. It is a general practice to never, ever get any service, of any type, except for Internet service from an ISP.
Because of that decision, you are now needing more IT knowledge than it takes to run a PBX. You've not saved any effort.
Why do you feel that VoIP is a headache? It seems to only be a headache because of your ISP. As someone who has run VoIP for over a decade, these kinds of problems didn't exist even back then.