PBX and SMS Server for GOIP

I have 4 people working from home including me, we do not have VPN, I do not have on-premise PBX system. However, I do have cloud PBX on Flynumber, Zadarma, and Bitrix24 for telephony, CRM, and everything else related to keeping the business running. A few months ago I purchased a GoIP4 SIP GSM Gateway, but been having a hard time configuring it, so it hasn't worked as expected up to now. I've got 4 local sim cards in the machine already but I never got them to work properly. I directed the calls to an SIP address on one of my virtual PBX, I want those sim cards to receive and make outgoing calls and I want all 4 people to be able to do so right from their stations at home even though we are not in the same network. At this point, I can receive calls but calls always get disconnected only after 10 to 15 seconds for unknown reasons. When I make calls, the calls are charged to my SIP provider instead of being charged to the SIM card, so I'm thinking this part must be configured locally in my machine. I do not have a dedicated server, it is in my understanding that the GoIP machine itself could act as a portable PBX or SIP server, correct me if I'm wrong. I want to focus first on making the calls work or if you can help set up the bulk SMS too, that would be great. I am open to any suggestions that would help me achieve my goal, that is basically to receive and make calls from and to the GoIP Gateway as this will help me reduced costs from my current SIP providers. I prepared a simple form below for those who would like to offer their assistance. I will appreciate anyone who would provide a solution and a fair-enough quote. I am willing to invest but please consider that I am only a small start-up business. Thank you very much.