I use Grandstream DP715, great little phone for the price, I think you can run 5 slaves off of one base station. Talk time is not bad, could be better. Easy to set up and configure the phones, each base station has a web server you configure your options from.
It’s a bit ‘left field’ but at one site, where there is full Wi-Fi coverage over several floors, a ‘cheap’ Android/smart phone which has built in SIP might be the way to go. I use a ‘Pay as you go’ Sony Experia (£55 from Vodafone) without a sim card as a handset.
I’m less trusting of the phones needing an App however.