My information is valid for UK, but I trust the process is similar in the US.

Below is what I would do in your place.
- I want to port a number from another provider to ringcentral
- I reach out to the other provider and get them to do 2 things:

1. transfer the ownership of the account from the old owner to me
2. ask them to confirm the address of the phone number with the number carrier. In other words your old provider is using a number carrier. The new VoIP provider - RingCentral - is also using a number carrier. The carriers are a separate entity from the VoIP providers.

- I submit a port-in request from the RC admin portal. The RC porting team will raise the request to the “old” number carrier so that the number gets ported over to the “new” number carrier used by RC. In order to avoid a rejection and a porting delay you need to ensure that you have the address of the phone number exactly as it is registered with the “old” carrier. This could be the same address that your old VoIP provider has on file, but not necessarily, hence the need to double check the address.

I think this is all you need to know in order to port your number. There is also an article about porting which you might find helpful:
https://support.ringcentral.com/article-v2/Submitting-your-Porting-order.html?brand=RingCentral&product=RingEX&language=en_US&pills-nav=submit_order_local