@charentejohn So nobody knows then ?   Does anybody know how I can contact tp-link themselves as there seems to be only the forums but these are not always populated by people who know the products well, more questioners than answerers ? :)

@ArcherC8 Thanks for replying.  That looks like the sort of thing I need.

Google Voice is one solution but that means trusting google will always support it, Shame TP-Link didn't add an RJ11 port like the Huawei box.

All I can think is that the other one has a small circuit inside that converts analogue to digital.

I am now looking at getting to the card via my mobile phone (different network) using wifi.   Still unsure how that would work as regards dialling a landline at the other end. 

So I phone with my mobile but tell it to use wifi not cellular mast.  So it then goes through the router and, here is the tricky bit, dials the number using the router sim not the phone one.  Logically it should be the router sim that makes the call, then again how do they call me back.  By dialling the router sim number ?

You can see why I am still trying to figure this out.

Editd to add that I am mainly asking as neighbours want to do this but need something simple, so just plugging in a phone is best for them.  So Huawei would suit them I think.  Form me I will persevere with the mobile to router option to get access to the sim in the TP-link, still strange they did not allow for this option as (unless a card is data only, never seen one) then the unlimited calls that accompany some sim cards here are wasted.

@charentejohn 

Here is a link to our TP-Link France Suppot Team: https://www.tp-link.com/fr/support/contact-technical-support/#Hotline-Support

I would definatley recommend contacting them as we do not have a product here in the states or sell one on our official US site for what you are looking for.

@Tony Thanks for the link I will get in touch with them. 

I don't know if TP-Link have a product that allows a phone to be plugged into it, if they don't it is a bit of an oversight.  Pretty much any 4G router will be using a phone type sim card and the inability to plug in a standard RJ11 phone is not good.

Again as it basically locks the sim card away inside the router shutting it away from it's function as a mobile phone card.  It has a mobile number assocaited with it but it can never be used to phone anyone.  Interestingly it can be used for texts, so why not calls ?

I will look into putting my mobile on the network in some way ?  Perhaps then I can dial via the sim card.

I have a Nokia windows phone that can (so it says) connnect via wifi so I assume it will allow calls out using the router sim card's package, so any charges (or not) will go through that.   No idea on calling back though, somebody at TP-Link should have thought about this though ?

In the meantime I will recommend my neighbours buy a Huawei 525 box as they need to have a simple system to use the internal card and this has that.   

@charentejohn Here is irony for you https://play.google.com/store/apps/details?id=com.tplink.tpphone

This says it is (despite poor reviews) a means of phoning via wifi and is an app from TP-Link.  I am still trying to download it (some links on the site broken) but it only seems to work for ADSL routers and not the MR600... why ?   This still means my sim card is in a block of cement inside the MR600 and inaccessible.  

Oh well I shall persevere, in the end I can always buy a Huawei 525 and use the MR600 as just an expensive router.

@charentejohn I am from India and also wanted to buy this product as it has cat6 technology support with 4g+ ,but i also want the voice call facility . Unfortunately it doesn't provide .And i am fortunate enough i got this message from you. Can you suggest me a good router having these facilities like 4g +,cat6,Voice facility. THANK YOU.