The "phone" is probably landline or old 2G phone, iphone is internet connected smartphone (extra whatsapp etc).

I remember having troubles voice calling iphone users, because of some roaming data bs, their device would not correctly fallback to 2g voice call on bad 4g connection, it tried to use data everywhere (voice call would just get ignored).

Plus imessage does not correctly use sms protocol, instead tries to use internet and apples bs extensions. With bad connectivity it means sms will not reliably work on iphones!

Super relevant in germany and their shitty 4g networks!!!