I loved every second of using it. So much cheaper and you can call and text from everywhere in the world as long as you have an internet connection. Being able to make calls on the desktop with a desktop xmpp client is great also. if you need help, they have a very reactive support and they also gave a chat room where the employees are very active. I used for about two years and then I stopped because some services require a physical SIM card phone number (they reject VoIP ones), and I could finally find a cheap enough plan to justify not using JMP anymore.
It's exclusively my phone number using Cheogram app from F-Droid. I paid for 2 numbers, one number is for bank and credit cards, I use that number to call financial places or verify login. My other number is my personal number and for Molly/Signal, etc.
Since JMP only requires XMPP, a federated service, it's essentially an anonymous phone service, and if you travel internationally, you connect to wi-fi in a different country and can make local calls to anyone back home.
All it is is a VoIP number, and some services detect a VoIP number and require a cell number, but you can port your cell number to JMP, you don't have to have a new number from JMP.
IIRC it's legally required to allow porting a number out of a service, but porting it in is usually something companies add as a nicety. It's good to hear they allow that.
Two questions, which are open ended:
Does porting a number out to them cause the number to get flagged as VOIP later? It would be really crappy for people if it did...
If you pick a JMP number, you can port your JMP to a cell service, and a cell number can be ported to JMP.
I believe, but I may be wrong, that cell/mobile companies get a different batch of numbers from VoIP companies, so if you port a cell number to JMP, it will likely still be believed to be a cell number, but I can't say 100% concerete for sure.
Yes, you can port out your jmp number, but it takes slightly longer than a regular port out/port in to the new provider because it is considered a landline. When I ported out of jmp, my number was accepted as a verification method, meaning it is no longer labeled as a VoIP number
I use Linux so my client of choice is Dino. If you are on Windows, I think there is Gajim that is recommended by JMP, and it is cross-platform, not windows or linux exclusive like Dino. This should also be of help: https://wiki.soprani.ca/ClientComparison