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There is a ton of ways to route the calls, one being the grandstream you mention. But a call from the US to Pakistan over standard internet would be nasty in quality. The Jitter and Latency would cause excessive echos, pops, or other issues with the calls. The owner of the number would be required to send the calls to your solution, and then the store / your staff would all need to be registered to that solution.

It sounds like a manual process. Why would you want to route the callers to your firm, when if using a IVR could do this and output a compatible HTML and CSS that can be displayed? Using a standard visual IVR is already doing this.

because I cannot promote any solution by name for this issue (Terms) I would suggest the 250.00 SIP PBX, as a local fix it. it has the station, trunk, paging and Hunt groups all built in with no additional licensing cost. You can still run the cloud accounts direct to the ISP, and the second lines can be accounts on the local SIP PBX.