Answer second call or make a call when busy

[spapakons]

Hello!

I'm searching your forum, but I don't seem to find what I need, so I decided to ask directly. I have a PC running Asterisk 2.4 and an OpenVOX B100P PCI ISDN BRI Card connected to an ISDN device with one main line and three MSN lines. I also have a Yealink SIP-T28P (6 lines) phone and two Tealink SIP-T20P (2 lines) phones. I have defined three SIP extensions, 510, 511 and 512. SIP-T28P is assigned extension 510 and the other two phones are assigned the other two extensions. I managed to configure Elastix so all phones ring at each incoming call. When I have a second call and one phone is busy, I can answer it from the other phones. What I want to do is to be able to put the first call on hold and answer the second call from the same device and select between the calls until I finish one of them. The SIP-T20P support two lines, so I want to be able to answer two calls from either of them while putting the first call on hold. The SIP-T28P supports 6 lines, so I want to be able to hold and reply at lest 2 and up to 6 calls. How can I do that? Please describe each setting/parameter in Elastix and the phones in detail, as I am a newbie and I cannot understand the relevant terminology.

In short, some time in the day I am alone in the office and I answer an incoming call at extension 511, say. While talking I hear a second incoming call. Of course, I don't want to run at the other end of the office to answer it, so I want to put the first call on hold and reply the second. I also want to be able to hold the second and return to the first if necessary. Another scenario is that I answer an incoming call and the customer asks for a product availability. How can I put him on hold and use the same device to call my provider?

Thank you for your time.