You will need to do a bit of Googling. There are a couple of possibilites. First, your phone settinjs may have that loaded in its settings. Second, your phone plan may have such a limit. This used to be common but these days unususl for relatively short calls like this (contact your phone carrier or look up your phone plan). Google "android limit phone talk time" for several suggestiond.
As @Troubadour said, this most likely related to your network plan. May we know who's your network provider? You might need to check it with them to see if there's any limit to your plan.
