I think the voicemail number will depend on whoever your fixed line provider is, though in most cases I believe it's 1571. For BT see here:

https://www.bt.com/help/landline/all-about-bt-answer-1571-voicemail

Other providers should have similar info available. Just do a search for "(provider name) voicemail number"

Here's a copy of your phone's manual

https://www.manualslib.com/manual/92280/Durabrand-Ib-Ph5595.html

Ohgr
November 2023