How to simplify the work with a lot of phone sims?

Question

SIM cards

Good day Dear.
Subject matter:
The manager uses three 2-SIM cell phones. In total, 6 inserted SIM cards are obtained. You ask why there are so many sim cards for one manager? I answer, these phone numbers are receiving calls from advertising. I thought how to simplify it all, so that there is only one phone, and many SIMs are connected, and here the thought comes: "Why not use call forwarding?" And because it's expensive. Lots of calls.
Dear, I am not strong in this area. Who has experience, advice, tell me.
What you need:
1. Are there options to have many phone numbers, but with free forwarding to one number.
2. Is it possible when the line is busy (the manager is talking to the client), the answering machine worked.
Thank you for your attention.

Answer 1

Install Asterisk with IP-GSM gateways and forward these calls however you like. It will cost, however, not cheap, the GoIP-8 gateway for 8 SIM costs about 32,000, but you can configure the call queue, the answering machine, and the recording as you like ...

Answer 2

variant of astcrisk with usb modems. They are cheaper and you can insert a lot.