Since most internet connections today are very good, you are free to choose which ever equipment or app you want to use as long as it supports SIP 2.0.
For equipment we do recommend using well known brands like Cisco, Fanvil, Gigaset, Grandstream, Linksys, Polycom, Snom and Yealink since lesser known brands often aren't reliable enough because they have buggy firmware and don't put enough ressources into fixing the bugs.
The easiest and cheapest is to use an app on your smartphone or computer and there are a lot of different options. Most will work if they support SIP and are not locked to a provider. Learn more
For private use or a small business, we recommend the cordless IP phones in the Gigaset Go series that can be used with both IP telephony and ordinary analog telephony, can handle 2 simultaneous calls and can be extended with up to 6 handsets. Learn more
If you have analogue telephones you are very happy with, you can use an analogue telephone adapter. If your internet connection is slow, it can be an advantage that the adapter has traffic prioritization (QoS) so that it can prioritize your call traffic higher and thus minimize interference. However, it only makes sense if all your traffic goes through the router and you do not f.x. use WiFi in the router you have from your ISP. Learn more
For business use, the best choice is often a PBX that come in a range of different sizes and features. Learn more
Can I use the IP telephony equipment I already have?
If you purchased your IP telephony equipment yourself, you can typically just change the setup and use it with Redspot. If your ISP has supllied your IP telephony equipment, it's only rarely possible - ask your ISP.