Your Phone
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Your answerphone message
Consider making your answerphone message more understandable to someone with dementia who may not realise that it's a recording of your voice.
For example "This is a recorded message on <your name>'s telephone answering machine. I'm not here to answer the phone at the moment, so please say a short message and I'll phone you when I get back. Hang up the phone when you've left your message.".
You may need to explain and practise the message with your loved one, and refine it if necessary (for example mentioning the beep).
Don't forget both your landline and mobile/cell phone.
Cordless landline phone, or headset for your mobile/cell phone
Having a cordless landline phone or a headset for your mobile/cell phone may be useful in enabling you to do things while on the phone to your loved one, especially if they phone you when you've started cooking.
If you use your mobile/cell phone for long calls to or from your loved one, consider getting a headset rather than holding the phone (and hence its radio transmitter) close to your ear - just as a precautionary measure, although no health effects have been proved.
Call diversion to your chosen phone
If it's less confusing for your loved one to have only one number to call you on, and sometimes you want that to be your mobile/cell phone rather than your landline (or vice versa), then it's usually possible to divert calls from one to the other. Note that call diversion will apply to everybody calling you, not just your loved one.
Charges
Do find out from your phone company what the charges will be. In addition to your loved one being charged for the call they make, usually you'll be charged for a call from the phone being diverted from.
A landline is likely to have high charges to call or divert to a mobile/cell phone, but diverting from a mobile/cell phone may use 'inclusive minutes'. Landlines in particular may also charge a monthly fee for the diversion facility, even if it's not used.
Switching diversion on and off
Usually switching 'call diversion' on is as simple as using the phone to be diverted in order to dial a code followed by the phone number to be diverted to. Switching it off is then just dialling another code (and you could store those numbers the same way you store people's numbers).
For mobile/cell phones, depending on your phone company and your phone, it may be possible simply to use your phone's settings menus to enter the number to be diverted to and then to switch diversion on or off.