Voice Gateway / POTS to GSM

Hey Guys, i may be in the wrong subreddit here but i didnt find one close to it as this one.

So:

I have a landline connection in a house, in the house there is only the landline no wifi or else.

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Now i want to make calls through that landline with my mobile phone even tho i am not there. Now there is the option with a voice gateway which plugs in the landline jack and connects to the internet (could be solved with a mobile router) and then i would be able make calls over the landline with my mobile phone.

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BUT:

I have a few more houses where i would like to do this, and besides the voice gateway beeing not that cheap (about 100€) and the need for a seperate mobile router, its not that convenient for me.

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Now my question is:

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Is there a device in which i can insert a sim card, and connect to the POTS/landline socket and then call that device with my mobile phone and it basically just routes the voice between my phone and the landline?

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That would be very practical.

Thanks alot!