Wireless intercom

Can anyone recommend a wireless intercom to speak to visitors after they have rung the doorbell? Sometimes I may be sleeping at the other end of the house and don't want to miss a delivery.
I don't need video or anything like that and I want to keep the price down.
I suspect a battery-powered receiver at the front door might not work for long, although a mains powered unit in the house may help this if such a product exists.
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