Is IP telephony possible with a regular SIM card?

Question

Telephony

Good afternoon.
I will describe the situation - a long time ago there was a company that decided that an ordinary SIM card from a mobile operator would be used as a company phone. Time passed and the disadvantages of this approach became full-length, but many customers remembered the old phone and when changing the number there was a risk of losing them. Attempts to call the numbers in the search engine results for the word "IP-telephony" did not give any unambiguous and intelligible solution.
The question is - is it possible to transfer to IP telephony by saving the number from the SIM card of that same ordinary mobile operator? Just at least a couple of words, knowing which one could start to google in the right direction.
Thank you.

Answer 1

Multiphone. Change of operator with the preservation of the subscriber number.